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Lithium Battery Consulting by Dr. K. M. Abraham

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          Timely Lithium Ion (Li-Ion) Battery Topics    

Ø Exploding Hoverboards Explained (And other lithium-ion battery devices)

Ø The Road From Solid Polymer Electrolytes to Commercial Lithium Ion (Li-Ion) Polymer Batteries

Ø What does Electrode Particle Size have to do with High Power Lithium-Ion Batteries?

Ø    Factors to Consider In The Design and Applications of High Power Lithium Ion  (Li-Ion)  Batteries

Ø    The Liquid to Polymer Electrolyte Continuum in Lithium Ion Electrolytes

Ø     Large Lithium Ion (Li-Ion) Batteries

Ø     Safety of Lithium Ion (Li-Ion) Batteries

Ø     Thermal Stability of Lithium Ion (Li-ion) Batteries

Ø     Rechargeable Batteries For The 300-mile Electric Vehicle and Beyond

                                    For brief discussion, click on title.  

 

Topics to come:

 Ø     Lessons Learned from Studies on Li Metal Anode Rechargeable Batteries for
               Understanding Lithium Ion (Li-ion) Batteries

Ø     The Role of Separators on the Performance and Safety of Lithium Ion (Li-Ion)
                Batteries

 Ø     Technological Superiority and Cost Considerations in the Use of Lithium Ion
                (Li-Ion) Batteries

 Ø     Lithium Ion (Li-Ion) Batteries with Advanced Cathodes and Anodes

 Ø    Overcharge and Over-discharge Protection of Lithium Ion (Li-Ion) Batteries

 

  The Road From Solid Polymer Electrolytes to Commercial Lithium Ion
             (Li-Ion) Polymer Batteries 

 Solid polymer electrolytes (SPEs) serve two principal roles in a Li battery.  It is the separator that insulates the anode from the cathode in the battery and it is the medium through which ions are transported between the anode and cathode during discharge and charge.  Consequently, the SPE should not only have high ionic conductivity, but it also should be an electronic insulator. In addition, it should have adequate mechanical strength to withstand the stack pressure changes and stresses of the electrodes during discharge/charge cycling of the battery.  Preferably, the mechanical strength of the polymer electrolytes should be comparable to that of the porous polyolefin separators used in conventional Li batteries utilizing organic liquid electrolytes.

 In general, conductivity and mechanical strength of a polymer electrolyte follow an inverse trend, i.e., mechanical strength decreases as conductivity increases. We recognized this behavior as early as 1988 when we attempted to fabricate rechargeable solid-state Li batteries with MEEP-based SPEs (1).  Although these classical polymer electrolytes based on poly[bis((methoxy ethoxy) ethoxy)phosphazene]  (MEEP) have room temperature ionic conductivities exceeding 10-5S/cm, they are glutinous materials with a tendency to flow under mild pressure. Consequently, Li batteries could not be constructed with these electrolytes unless their mechanical strength was increased.  We developed two approaches to prepare mechanically strong MEEP electrolytes.

In the first approach, mechanically strong electrolytes were prepared by forming interpenetrating polymer network (IPN) structures and composites with morphologically rigid polymers such as poly (ethylene glycol diacrylate) (PEGDA), poly (vinyl pyrrolidinone) (PVP),  poly(ethylene oxide) (PEO), or poly(propylene oxide) (PPO) (2).  In the second approach we encapsulated MEEP electrolytes in thin, highly porous, fiber glass separator mats (1).  A Li cell of the configuration, Li/MEEP-(LiX)-fiberglass/TiS2, was constructed and cycled at room temperature.  This represents one of the early demonstrations of room temperature performance of conventional solid polymer electrolyte-based rechargeable Li batteries.  Because of the relatively low conductivities, however, these solid polymer electrolytes, derived from Li salt complexes of long chain polymer hosts, are not suitable for the development of Li batteries capable of high power performance at and below room temperature

 Solid Polymer electrolytes with conductivities exceeding 10-3 S/cm at room temperature have been prepared as gel electrolytes (3,4). They are formed by absorbing organic liquid electrolytes into polymer structures.  Examples include gel electrolytes obtained by immobilizing LiPF6 solutions in mixtures of ethylene carbonate (EC) and propylene carbonate(PC) in polymers such as  polyacrylonitrile (PAN), poly(vinylidene fluoride) PVF), poly(methyl-methacrylate) (PMMA), poly(vinyl pyrrolidinone) (PVP) and poly(ethylene glycol diacrylate) (PEGDA)-based electrolytes(3,4).  The mechanical strength of these gel electrolytes is usually adequate for use as separators in Li-ion batteries, but their soft morphology often is unsuitable for the high speed processing employed in large volume manufacturing of batteries.  The flow properties of some of these electrolytes can also lead to internal short-circuits and safety hazards.

 In order to develop highly conductive polymer electrolytes which can be handled like conventional separators, we set out to  prepare microporous membrane-bonded polymer electrolytes. As an example of this concept, we encapsulated PEGDA-based gel electrolytes into the pores of several types of Celgard® membranes(5). We found that ionic conductivities of these polymer electrolytes can be modulated by controlling the thickness and porosity of the separator.  This was an extension of the work we had carried out previously with unsupported PEGDA-based electrolytes having the compositoin of PEGDA-EC/PC-LiX (LiX=LiClO4, LiAsF6) with a room temperature
conductivity of about 3x10-3 S/cm (4).

 Conductivities of a few of these separator-bonded electrolytes are shown in Table 1. The PEGDA-based electrolytes were formed by in-situ polymerizing the tetra-ethylene glycol diacrylate (TEGDA)-containing electrolyte solutions on Celgard membranes. The pores of the membranes were fully covered by the solid electrolytes.  As expected, membranes with higher porosity gave electrolytes with higher conductivity.  In addition to good mechanical strength, the resulting electrolyte showed excellent electrochemical stability with a voltage window spanning 0.0V to 4.3V versus Li+/Li.  Li/LiMn2O4 cells using these electrolytes as the separators  showed excellent cycling behavior at room temperature (5).

 Incorporation of polymer electrolytes into porous separator membranes lowers their conductivity to a degree determined by the solid fraction of the membrane. However, mechanically strong thin electrolyte films can be prepared which are useful to the construction of rugged batteries. The conductivity loss of the electrolyte due to the porous membrane is compensated for by its smaller thickness. It should be noted that in this work we used commercially available separator membranes to demonstrate the concept.  By suitably optimizing the thickness and porosity of the porous membrane, it should be possible to prepare solid polymer electrolytes with conductivities comparable to that of unsupported polymer electrolytes, (i.e., without the membranes). It is not necessary to fully cover the pores of the membrane with the polymer electrolyte. A fraction of the pore may be left open to store liquid electrolytes to provide high ionic conductivity in the cell. The polymer electrolyte layer can be used to bond the separator to the electrodes to achieve a monolithic cell package suitable for the construction of thin prismatic batteries with dimensional stability.  

  Various modifications of this concept have been adopted to fabricate practical batteries which are being introduced commercially.  Das Gupta and Jacob (6) reported on Li-ion batteries with bonded electrolytes in which a microporous separator and electrodes were coated with a liquid electrolyte solution and a layer of a polymer adhesive plasticized with a liquid electrolyte solution.  The separator and the electrodes were then bonded.  Another variant is a process in which an adhesive layer (e.g. PVdF) is applied to the separator and used to bond the electrode and separator films (7,8).  Sony described liquid electrolyte plasticized polyacrylonitrile layer directly applied either to the electrode or the separator (9).  Note that practical utility of polyacrylonitrile-based electrolytes for Li and Li-ion batteries were reported by us in many publications since 1990 (3,4,10).  In Sanyo’s work (11) on separator bonded electrolytes, they added polymer precursor solutions to spirally-wound liquid electrolyte-based Li-ion cells and solidified the cell package by thermal polymerization.  Researchers from Telecordia technologies (12) described a process wherein propylene carbonate-plasticized electrodes were permanently bonded to several different untreated microporous polyolefin separators.  Batteries fabricated using this process reportedly exhibited excellent performance:  3 C rates with 75-80% capacity utilization at room temperature and 50% capacity at C/2 at –20oC, very low internal impedance and high specific energy (>180 Wh/kg).

  In conclusion our work to improve the mechanical strength of gel polymer electrolytes by bonding them to micro-porous membranes has been transitioned into a successful technology for the fabrication of commercial Li-ion polymer batteries.

 

References:

 1.  M. Alamgir, R. K. Reynolds and K. M. Abraham in “Materials and Processes for Lithium Batteries,” The Electrochemical Society, PV 89-4, K. M. Abraham and B. B. Owens, Editors, The Electrochemical Society, Pennington New Jersey, (1989), pp. 321.

2.  K. M. Abraham and M. Alamgir, Chem. Mater. 3, 339  (1991).

3.  K. M. Abraham and M. Alamgir, J. Power Sources, 43-44, 195 (1993).

4.  K. M. Abraham and M. Alamgir, J. Electrochem. Soc. 136, 1657 (1990) (See also U. S. Patent 5,252,413 (1993).

5.  K. M. Abraham, M. Alamgir and D. K. Hoffman, J. Electrochem. Soc.  142, 683  (1995).

6.  U. S. Patents 5,437,692 (1995); 5,498,489 (1996) and 5,512,389 (1966).

7.  G. Venugopal, J. Moore, J. Howard and S. Pendalwar.  J. Power Sources, 77, 34 (1999).

8.  U. S. Patent, 5,981,107 (1999), 6,024,773 (2000) and 6,051,342 (2000).

9.   K. Kezuka, T. Hatazawa and K. Nakajima in Proceedings of the 10th IMLB, Como, Italy, May, 2000 (Abstract # 4).

10.   K. M. Abraham, H. S. Choe and D. M. Pasquariello, Electrochim Acta. 43, 2399 (1998)

11.  T. Fujii, Proceedings of the 17th International Seminar and Exhibit on Primary and Secondary Batteries, Boca Raton, Fl, March, 2000.

12.  A.S. Gozdz, I. Plitz, A. DuPasquier and T. Zhang.  In Rechargeable Lithium Batteries, The Electrocheical Society Proceedings Volume, PV 2000-21, pp 336, 2001

 

Table1.  Conductivities of Porous Membrane-based Solid Electrolytes at 25oC

Membrane                                        Electrolyte                                                                  Thickness        Conductivity 

Porosity   Thickness                                                                                                             ( micron)            ( ohm -1 cm-1)

(%)        ( micron)                                                                                                                       

70               25                 30(m/o)EC-30 PC tetraglyme-2 PEGDA-8 LiAsF6                         37.5         1.1 x 10-4         

55               25                74(m/o) EC-16 PC-2 PEGDA-8 LiAsF6                                              40.0          1.7 x 10-4               

35               25                74 (m/o) EC-16 PC-2 PEGDA-8 LiAsF6                                             40.0           3.0 x 10-5         

34               25                30 (m/o) EC-30 PC-30 tetraglyme-2 PEGDA-8 LiAsF6                  37.5           1.2 x 10-5         

42               25                30 (m/o) EC-30 PC-30 tetraglyme-2 PEGDA-8 LiAsF6                  37.5          1.7 x 10-5         

-------------------------------------------------------------------------------------------------------------------------------------------------------                                            

     Unsupported   Electrolyte:     3(m/o) PEGDA-68 EC/15 PC-14 LiPF6                         100              3 x 10-3

 

For an in-depth discussion of Li-ion polymer batteries and solutions to practical problems, consult Dr. K. M. Abraham at E-KEM Sciences   

 

 

2.  Factors to Consider In The Design and Applications of High
    
Power Lithium Ion (Li-Ion) Batteries

 High Power Li-ion batteries capable of being fully discharged and charged in several seconds to a few minutes are needed for hybrid electric vehicles, directed energy weapons, power tools and many other high power applications. The selection of appropriate electrode and electrolytes materials and porous separator, and the design of optimum electrodes, current collectors, cell stacks and battery packs with low resistances and polarization losses in order to sustain the very high current drains are key to the successful development of such batteries.

The power, P, of a Li-ion cell is given by P=V2/R, where V is the cell’s load voltage and R is its internal resistance, high voltage cells with low internal resistances are the most desirable for high power batteries. In additional to the polarization losses associated with the resistances of the various cell components, mass transport in the solid electrode lattice and in the electrolyte solution/separator phase have to be understood and optimized to obtain the highest continuous power output from the cell. A pragmatic methodology to design such batteries is needed. It can be shown that an electrode material with a relatively high solid-state Li diffusion coefficient of the order of 10-10 cm2/sec (e.g., LiCoO2 and Li2Mn2O4) can be fully discharged in 100 seconds using about one micron size particles. On the other hand, if the diffusion coefficient of the electrode material is a much lower 10-14 cm2 /sec, (e.g., LiFePO4) approximately ten nanometer size particles are needed to achieve full discharge in the same time period. The use of very small electrode particles having the latter dimensions may introduce potential issues such as difficulty to fabricate low impedance electrodes, enhanced electrode surface reactions with the electrolyte leading to the loss of active material and increases internal resistances, and penetration of particles through the separator pores to cause high rates of self discharge. An awareness of the latter problem is especially important as separator porosity and electrolyte conductivity show a direct relationship to the discharge rate capability of high power Li-ion cells (1).  Rationally selecting both active and inactive materials and incorporating them in optimally designed electrodes, and building cell stacks with minimal internal resistances and optimal ion transport profiles to produce high power Li- ion batteries are key to the development of high power Li-ion batteries (1-3).

 Small high power Li-ion cells with LiMn2O4, LiFePO4 and LiMn-Ni-oxide cathode materials are commercially available. Interestingly, high power Li-ion batteries are not fabricated with LiCoO2. What is the reason?

 For an in-depth review and discussion of high power Li-ion batteries, including which Li-ion battery to choose for an application; performance, safety and life issues of high power Li-ion batteries for power tools; and, applications of large format high power Li-ion batteries for hybrid EV consult Dr. K. M. Abraham at E-KEM Sciences   

 

    References:

 1.  K.M. Abraham, D.M. Pasquariello and E.M. Willstaedt; Discharge rate capability of LiCoO2 electrode, J. Electrochem. Soc., 145, 482-486 (1998)

2. K.M. Abraham and M. Alamgir, "Solid-State Carbon/LiNiO2 Pulse Power Batteries", in Proceedings of the 36th International Power Sources Symposium, published by IEEE, New York, NY (1994), p. 257.

3. H.S. Choe and K.M. Abraham, “Synthesis and Characterization of LiNiO2 as a Cathode Material for Pulse Power Batteries”, in Proceedings of the Symposium, Materials For Electrochemical Energy Storage and Conversion II - Batteries, Capacitors and Fuel Cells, Fall  MRS  Meeting, Boston, Ma. December 1-5, 1997.

 

 

 

3.  The Liquid to Polymer Electrolyte Continuum in Lithium Ion (Li-Ion) Battery Electrolytes

 The electrolytes influence the rates of discharge, low temperature performance, high temperature stability, and cycle and calendar life of lithium and lithium ion batteries. Consequently, knowledge of the ion transport mechanisms in electrolytes is important in designing new and improved electrolytes. Our recent results show that there are strong similarities between the ion transport processes in Li ion conducting liquid and polymer electrolytes so that there is a limit to the conductivity that can be achieved for conventional solid polymer electrolytes. Our data also have provided insights into the design of improved polymer electrolytes (1,2).  

 We measured the conductivities (K) of solutions of LiPF6, LiN(SO2C2F5)2, LiBF4 and LiC(SO2CF3)3 in a variety of carbonate solvents as a function of temperature (T). The conductivity data from – 40 to +85 oC provided insights into the ion conduction mechanism in Li-ion conducting electrolytes. The K versus 1/T plots fit well to the Vogel-Tamman-Fulcher (VTF) relationship shown in equation 1 describing the dependence of ion conductivity to free volume in solution. In this equation, A and B are constants; A is a pre-exponential function, B is the energy needed to create free volume for ion movement in solution, and To is the temperature at which solvent structural relaxation becomes nonexistent. 

   K = AT-1/2 exp{-B/T-To}                                                                         (1)              

The closeness between the measured glass transition temperature Tg of the solutions and their To values calculated from conductivity data provides strong support for solvent coupled motion of ions in Li-ion conducting electrolytes. The solvent-assisted transport of ions in Li battery electrolytes is further supported by 13C NMR data that show that the Li-ion are solvated by the solvent molecules and their mobility is coupled to the mobility of the solvent molecules.                  

 The conductivity-temperature behavior of the liquid electrolytes we have observed is similar to the well-established behavior of Li-ion conducting solid polymer electrolytes. The polymer segmental mobility assisted mechanism of ion transport in polymer electrolytes is well recognized. This suggests that ions conduct via similar solvent–assisted mechanisms in both liquid and solid polymer electrolytes.

  A good understanding of the ion transport mechanism in liquid and polymer electrolytes has allowed us to design improved electrolytes with superior low temperature performance, high temperature stability, and long life in Li-ion and Li-ion polymer batteries

 For an in-depth review and discussion of electrolytes for Li-ion and Li-ion polymer batteries, consult Dr. K. M. Abraham at E-KEM Sciences   

References:

1. G.Y. Gu, R. Laura and K. M. Abraham, Electrochem. and  Solid-State Lett., 2, 486 (1999).

2. B. Ravdel, K.M. Abraham, R.L. Gitzendanner and C. Marsh, in Batteries and Supercapacitors, The Electrochemical Society Proceeding Volume, PV2001-21 (2003)

 

  4.  Large Lithium Ion (Li-Ion) Batteries

 Since their introduction in 1990, Lithium-ion batteries have enjoyed spectacular market growth as the power source of choice for portable consumer products such as cellular telephones, personal digital assistants, video cameras and notebook computers (1). Commercially available 18650 small cells have a specific energy of 180-200  Wh/kg, an energy density of 400-450 Wh/liter and yield 500-1000 full depth charge/discharge cycles. The wide-spread consumer acceptance of these rechargeable lithium batteries can be attributed to their superior performance characteristics, which include higher energy density, longer cycle life, better low temperature performance, lower self-discharge rate and longer shelf life, compared with the traditional Lead-Acid and Nickel-Cadmium rechargeable batteries. 

 Almost all of the applications of Li-ion batteries are currently filled by small cells, prominently the 18650 size cells having a capacity of ~2Ah, and small batteries constructed by the series/ parallel combination of a few of them. Despite their superiority in practically every aspect of performance of a battery, Li-ion batteries have not penetrated the high-capacity market dominated by Lead-Acid for applications such as backup power systems because of higher cost and safety concerns. Even for applications that are not cost sensitive, potential safety concerns have remained as a deterrent to the widespread use of large Li-ion batteries with hundreds to thousands of watt-hours (Wh) of stored energy. Traditionally such large Li-ion batteries have been constructed from the series/parallel stacking of large monolithic cells, each with tens to hundreds of ampere-hours (Ah) of capacity. Large monolithic Li-ion cells are more prone to safety hazards because they are unable to quickly dissipate the internally generated heat from such abuses such as internal short circuit and overcharge to prevent pressure buildup and the associated cell rupture and venting. Large cells also tend to be more expensive because of higher manufacturing costs.

Small commercial Li-ion cells with a capacity of 1-2 ampere-hours, such as the 18650 cells, have several layers of protection against hazards from short circuit and overcharge, and that account for their widespread consumer acceptance. High volume manufacturing has lowered the price of small Li-ion cells to levels very attractive for high energy applications such as telecom and cable TV backup power. What has been lacking until now is a reliable way to build large batteries from small cells.

Modular Energy Devices, Inc. of Westerly, RI has developed Li-ion battery modules and packs with capacities ranging from 80 to 200 Ah for Cable TV/Telecom power backup applications. The modules are constructed from small, highly reliable and safe 18650 Li-ion cells using the proprietary electronic technology called Massively Parallel Modular Architecture. The battery modules are readily combined in series and parallel to produce 24, 36 and 48 and 96 V battery packs with appropriate capacities for cable TV/Telecom applications. The use of small cells in combination with the superior electronic architecture makes the batteries safe, reliable and low cost.

  There are increased efforts to use large Li-ion batteries for aerospace applications in satellites, military and commercial airplanes, unmanned aerial vehicles and many others. The most famous recent aerospace application of Li-ion batteries is in the Mars Rovers, Spirit and Opportunity, which significantly exceeded the pre-launch life expectations. The 28 V Li-ion batteries performed more than one year on the Martian surface.  Large Li-ion batteries are also developed for many other civilian and military applications. In all of these concerns arise about their performance, safety, life and temperature-dependent performance.  

  For additional discussion and consulting on large format Li-ion batteries constructed from small and large cells including their performance, safety and applications consult Dr. K. M. Abraham at E-KEM Sciences

 Reference:

K. M. Abraham, Stephen Eaves and Farshid Bhaktyari, “Low Cost Lithium-Ion Batteries for Cable TV/Telecom Power Backup”, in Proceedings of the 2004 Intelec Meeting, Chicago, September 20004

  

 

5.    Safety of Lithium Ion (Li-Ion) Batteries

The dense packing of energy makes Li-ion batteries susceptible to safety hazards when abused. Of particular concern are conditions that dramatically increase the internal temperature of the cell. Two conditions that are potentially unsafe are short circuit and severe overcharge.

Short Circuit of Li-Ion Batteries: It is obvious that short-circuiting an energy dense battery cell will increase its internal temperature to high values in a relatively short time. Manufacturers of small 18650 Li-ion cells have been able to overcome safety hazards from short circuit through several design features of the cell. The key to the design of a safe cell is to incorporate features that would prevent the buildup of excessive heat that can vaporize the organic solvents in the cell, resulting in internal pressure build-up, cell rupture and venting.  Generally this problem is mostly attributed to large Li-ion cells rather than 18650 cells, since the large cells are unable to quickly dissipate the internally generated heat and prevent pressure buildup.

 Small cells such as the 18650 Li-ion cells with a capacity of about 2 ampere-hours have several layers of protection against hazards from short circuit. First of all, the small size of the 18650 cell enables the heat generated during a short circuit to be dissipated fast enough from the inside to the outside to prevent buildup. In addition, the cell incorporates a current limiting device, referred to as a PTC (positive temperature coefficient), that erves as a reversible thermal fuse and over-temperature protection device.  As a back-up to the PTC, shut-down separators and/or current interrupting devices (CID) can permanently limit current flow during internal short circuit and prevent excessive pressure build-up and cell rupture. If all of these safety features fail the cell incorporates a pressure relief disc to safely vent the gases generated. The gases generated are non-toxic organic compounds, hydrocarbons and carbon dioxide. 

Overcharge of Li-Ion Batteries: A Li-ion cell is usually charged to a potential of about 4.2 V. The cell generates very little heat during this normal charging process. Experiments have revealed that it can be safely charged to a potential of at least 4.5 V without excessive heat buildup. Excessive heating of and pressure buildup and heating in the cell can occur if the voltage exceeds 4.5 V and hovers around 5 V.  Such a severe overcharge of Li-ion cell leads to the plating of elemental Li on the carbon anode and the formation of chemically unstable cobalt oxide. Our work indicates that the safety hazard upon overcharge does not originate from the plated Li but from the unstable cobalt oxide cathode although the plated Li on the anode may have a role in exacerbating the ensuing runaway reaction. A cell would encounter this situation if the charger fails and the electronic circuitry in the cell fails. In a battery built with cells connected in series, overcharge can occur if each of the cells is not electronically protected against this condition.

 The 18650 cells are safeguarded against excessive heat buildup from the rare event of an overcharge by means of all of the above features that are also used to protect it against short circuit. When these cells are combined to form batteries, each cell is further protected against overcharge with electronic circuitry.  Large Li-ion batteries may not contain all the necessary safety features, and also suffer from the basic problem of using large monolithic cells for which adequate internal protection mechanisms have not yet been developed. When such large batteries experience a safety hazard, there is a tendency to blame Li-ion batteries as a whole.

There is no question that Li-ion batteries can be unsafe if they are abused.  The majority of incidents occurred in the early days of portable equipment.  This was because the early cells did not have all of the safety features of today’s cells.  Other problems may occur in battery packs that incorporate poor electronic protection circuitry, or if the pack is exposed to mechanical abuse, such as puncturing a cell with an electric drill or fork lift truck.

For additional discussion and consulting on the safety pf Li-ion batteries consult Dr. K. M. Abraham at E-KEM Sciences

 References:

1. S.N. Iaconetti, B. Ravdel, S. Santee, K. M. Abraham and J. F. DiCarlo “Overcharging Characteristics of 7 Ah Prismatic Lithium-Ion Batteries”, in Proceedings of 40th Power sources Conference, Cherry Hill, NJ  2002, pp 37

2. C.L Campion, W. Li, W.B. Euler, B. L. Lucht, B. Ravdel, J.F. DiCarlo, R. Gitzendanner and K. M. Abraham, “Suppression of toxic Compounds Formed in the Decomposition of lithium-Ion Battery Electrolytes”, Electrochem. and Solid-State Lett., 7, A194 (2004)

 

 

6.  Thermal Stability of Lithium Ion (Li-ion) Batteries

The calendar life and cycle life of Li-ion batteries are sensitive to the environmental temperature. An examination of commercial 18650 cell specifications reveal that all cells do not behave in the same manner. Typically, a Li-ion cells shows two types of capacity loss on storage at elevated temperature; a reversible capacity loss and an irreversible capacity loss. Interestingly, the values of each of these capacity fades are manufacturer dependent suggesting that they are both construction and material dependent. Similarly, the capacity fade rate on charge/discharge cycling of 18650 Li-ion cells can also vary from manufacturer to manufacturer.  Our recent studies of Li-ion cells with reference electrodes have provided insight into the origin of these capacity losses

 The traditional view has been that Li-ion battery degradation is largely due to thermal decomposition of the electrolyte. However, our recent investigations have shown that this is not correct. In this context we studied the thermal stability of lithium-ion battery electrolytes composed of LiPF6 in a mixture of organic carbonate solvents.  The generation of PF5 via the thermal dissociation of LiPF6 and its conversion to POF3  in presence of moisture or alcohol impurities  leading to an autocatalytic decomposition reaction is the primary source of thermal decomposition.  It was found that the addition of low concentrations (3 –12 %) of Lewis basic additives provides a dramatic increase in the thermal stability of the electrolyte.  The increased stability is linked to the formation of base: PF5 complexes which reversibly sequester PF5 preventing the decomposition of the electrolyte.  Lewis bases investigated include pyridine, hexamethoxycyclotriphospazene (HMOPA), and hexamethylphosphoramine (HMPA). We hae found that our results can serve as a valuable model for stabilizing organic electrolytes against thermal decomposition initiated by Lewis acids.  WE have also found that LiPF6 solutions in mixed organic carbonates do not decompose when stored at 85°C in the presence of charged or discharged cathodes such as lithium cobalt oxide and lithium nickel-cobalt oxide. The cathodes stabilize the electrolytes apparently scavenging the impurities that catalyze the reaction. Thus, homogeneous chemical reactions in the electrolyte solution do not appear to be the ultimate cause of Li-ion cell thermal degradation. The most significant contribution to the cell’s capacity and power fade upon storage seem to come from chemical reactions at the electrode-solution interfaces.

  The question arises whether an accelerated test, either cycling or storage of Li ion cells at elevated temperature,  can be used to predict life at room temperature. This author has formed some definite opinion on this from studies both commercial and custom made li-ion batteries. It is first of all useful to examine the theory behind accelerated tests.

 The accelerated test is usually based on the Arrhenius relationship and relies upon the assumption that the logarithm of rate of capacity fade, K, is proportional to the inverse of temperature, T (in Kelvin).

                                        K   = A. e-E/RT                                          (1)

                 Or,                 log K =   log A  – E/ 2.303RT                  (2)

 Here, A is a proportionality constant, E is the activation energy for the capacity fade reaction and R is the gas constant, 1.987. According to equation 2, a plot of logarithm of capacity fade rate versus 1/T will give a straight line from which the activation energy can be determined which then can be used to determine the life at an unknown temperature.  It should be noted the fade reaction mechanism is assumed to remain the same over this temperature range

 First of all we can show that that for an activation of energy of ~15 kCal/mole, the life of a cell would double for a 10 degree decrease in temperature.  For this we differentiate equation 2 with respect to temperature and integrate between two temperature limits, giving

 Log   K2        =         E      (T2-T1)   (3)
            K1            2.303 R   (T1T2)

It can be shown that the value of E, the activation energy, determines whether the Li-ion battery capacity fade will double, triple, or follow some other order, for every 10 degree rise in temperature. We can put in some numbers into equation (3) to see this outcome.

For the two temperatures of 40 and 50 oC with an activation energy, E, of 15 k Cal (15,000 Cal) /mole the fade rate will approximately double when going from 40 to 50 oC. (Note temperature is in Kelvin). Thus,

 Log      K50        =       15,000 (323 – 313)
              K40                  2.303 x 1.987 (323x313) 

                          =  150,000            = 0.32
                              462635  

 Therefore, K50       = 10 0.32  = 2.1
                    K40

 That is the fade rate at 50 oC will be about 2.1 times faster than that at 40 oC for an E of 15,000 Cal /mole or conversely the life at 40 oC will be about twice as long as the life at 50 oC. If the activation energy is higher than 15 kCal/mole, the fade rate will increase more than twice from 40 to 50 oC or the life at 40 oC will be more than twice that at 50 oC.  It is clear that a very good understanding of the relevant capacity fade reactions and their kinetics are needed for a comprehensive understanding of the thermal stability of Li-ion cells.

For a comprehensive discussion of how temperature influences the performance of Li-ion batteries and to find out if the common rule of thumb principles employed in the accelerated testing of conventional batteries can be extended to Li-ion batteries, and to ascertain what studies should be carried out to determine and predict Li-ion battery life, consult Dr. K. M. Abraham at E-KEM Sciences

 

References:

1. J.S. Gnanaraj, R.W. Thompson, S.N. Iaconneti, J.F. DiCarlo and K. M. Abraham, “The Formation and Growth of Surface Films on Graphitic Anode Materials for Li-ion Batteries” Electrochem. and Solid-State Lett., 8, 128,  2005).

2. W. Li, C.L Campion, B. L. Lucht, B. Ravdel, J.F. DiCarlo, and K. M. Abraham, Additives to Stabilize LiPF6-Based Electrolytes Against Thermal Decomposition ”, J.         Electrochem. Soc., 152, 1361(2005)

 3. B. Ravdel, S.A. Trebukhova, K. M. Abraham and J. F. DiCarlo, “Coin Cell with Two Reference Electrodes for Fundamental Study of Lithium-Ion Cells”, Fall Meeting of The Electrochemical Society, Los Angeles, Ca. ,  September 2005. Abstract No. 246

 

 For Comprehensive Information on All Aspects of Lithium Ion (Li-Ion) Batteries

                                              Consult E-KEM Sciences

 
   

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