Next-Generation Chemicals for
Lithium Rechargeable Batteries

W. F. Howard, Ph.D.
Edmond, Oklahoma

Potential materials for next-generation lithium rechargeable batteries are related to today’s technologies. Comparisons address battery systems, including electrolytes and anodes, acknowledging that industry’s desire for higher energy and greater power originates with the cathode. Qualitative and, where possible, quantitative ratings are listed in terms of capacity/energy output, stability, safety, environmental impact, and cost. Materials’ strengths and weaknesses are contrasted to existing or presumed battery specifications. The objective is to preview advances in rechargeable lithium technology, suggesting avenues ripe for development into marketable materials.

Introduction

Sony first produced Li-ion batteries for portable devices in 1990, using the now familiar system of LiCoO2 (LCO) cathode, LiPF6/alkyl carbonate electrolyte, and graphitic anode. The technology has undergone relatively minor changes since then, most notably the replacement of 70 to 80% of the Co with Ni, a move that substantially increases capacity and improves overcharge stability. Other promising cathode materials still under technical or marketing development include spinel (LiMn2O4) and Li iron phosphate (LiFePO4): both offer cost and safety advantages over LCO. Alternate electrolytes suffer from high voltage degradation, especially conductive polymers. One salt with the necessary stability is Li bis (oxalatoborate), LiBoB, but this has been slow to win acceptance. For safety reasons, only carbon anodes are widely used: lithium and Li alloys are deemed too active for consumer batteries in today’s litigious society.

Applications for rechargeable lithium batteries grew little beyond hand-held devices until legislation in the mid-1990s promoted electric and hybrid vehicles. Other uses for large stationary units have emerged, most notably Avestor’s lithium metal polymer battery for telecommunication backup. Battery size expansion presents formidable obstacles, especially economic (cost of new technologies) and engineering (thermal management for safety). Also, different applications will have unique requirements for battery performance, thus requiring different cell chemistries to meet specifications. This article will focus on next-generation materials and how their development will impact the lithium rechargeable battery industry.

Materials in a rechargeable cell do not perform independently. A high voltage cathode material will require an electrolyte stable to 4.5V or above, while pulse charge/discharge systems may need very small electrode particles and a liquid electrolyte. The discussion below is predicated on that interdependence: it does no good to advocate high performance electrodes without a robust electrolyte capable of sustaining the technology.

Cathode Materials

Looking first at the old standard, LiCoO2 has undergone numerous modifications to enhance capacity (replacing Co with Ni) and stability (modest Al levels suppress deoxygenation during charge). The driving force leading the industry away from LiCoO2 derivatives is the volatile (and rising) cost of cobalt, a strategic metal. Since December, Co prices have more than doubled, and even Ni has risen sharply. The automotive industry is very aggressive about minimizing costs and will not look favorably at increased battery prices, whatever the reason. Despite Nissan’s use of LiCoO2-based batteries in some of their HEVs, it is unlikely that other manufacturers will emulate this approach. Sanyo is already blending spinel with Li(Co,Ni)O2 to reduce costs and improve safety, albeit with a modest performance decline. With so much scientific attention already focused on LCO derivatives, it is unclear what advances remain: perhaps a combination of dopants will push the thermal decomposition point above 300°C.

LiFePO4 (LFP) has an exceptionally flat discharge plateau at 3.4V, especially suitable for complex circuitry, with moderate capacity (150-160mAh/g) and very low fade and toxicity. Performance is sensitive to impurities, especially Fe+3, and quality control will be a major issue for the suppliers. In fact, this could negate the apparent cost advantage of LFP over competing materials. If high purity precursors are required, even a common starting material like Fe2O3 is expensive. There is still some question about capacity retention at high discharge rates, although this could also be a function of phase purity. Finally, LFP achieves optimum output at 50°C or above, which may rule out conventional solution electrolytes.

These attributes suggest LiFePO4 is best suited for EV batteries and other applications with modest discharge rates, unless a cost-competitive LFP nano-material becomes available. Because LFP is charged to roughly 4V, there is not an immediately suitable polymer electrolyte. Valence Technology’s Saphion® battery, with Bellcore-type electrolyte, performs well at below C rates. Other LFP proponents include Armand and Chiang, and the commercial field is becoming increasingly competitive.

Simple LiMPO4 olivines (M not Fe) are difficult to prepare and have limited capacities, albeit with high discharge potentials and thus, high energy output. More complex olivines, such as Li3V2(PO4)3 (Valence Tech- nology), offer good cathode stability at less than 4V. The related NASICON materials, first proposed by Goodenough, have not achieved industrial acceptance.

Lithium manganese oxide, as Li1+xMn2-xO4, (spinel or LMO) offers numerous advantages over competing cathode materials: low cost, high thermal threshold, excellent rate capability, and minimal health and environmental impacts. But LMO is severely disadvantaged by the instability of Mn+3, the electrochemically-active ion, in the presence of acid, and reversible capacity is less than desired. The battery industry’s insistence on replacing LCO with spinel in Li-ion units is akin to pounding a square peg into a round hole. The trace acids accompanying LiPF6 and generated by alkyl carbonate degradation above 4V dissolve Mn+2 from the cathode and cause excessive fade. Should the spinel be modified or the electrolyte changed?

Creative scientists have put forward many protective mechanisms for spinel, without achieving LCO lifetimes. There are two preferred approaches: an acid-resistant coating (getter) on the LMO particles, or a modifier (dopant) to suppress Mn+3 disproportionation. Neutralizing the acid has an inherent weakness: each charge cycle generates a small quantity of H+ from the electrolyte, which eventually must overcome the protection. (Note that this includes basic electrolyte additives.) Amphoteric or ceramic metal oxides are typical coatings that must tread the fine line of complete particle coverage without restricting Li+ transport to and from the spinel. E-One Moli Energy patented this type of spinel for commercial batteries.

Substituting Li+ and various M+2/+3 cations for Mn+3 extends LMO cycle life to several hundred cycles at the expense of an already-low capacity. Aluminum is particularly effective, enhancing rate capability if separate Al2O3 phases are present (Howard). Pacific Rim companies produce a single-phase, low surface area LMO (<0.3m2/g) containing stabilizing Al, and report >1000 cycle performance. Spinel and battery manufacturers yearn for the development of an acid-free salt and/or polymer electrolyte with good ionic conductivity, and widespread industry acceptance of LiMn2O4 likely depends on such a discovery.

The layered lithium manganese nickel oxides (LMN), with multiple Li:Mn:Ni ratios, structurally resemble LCO, but do not de-oxygenate when charged (re­duced fire/explosion risk). (See recent papers by Dahn, Amine, and Ohzuku for examples.) Although LMN will cycle to >4.5V, delithiation generally results in reduced conductivity and therefore rapid electrolyte solvent degradation. Both energy and power performance improve significantly above room temperature. Further, these compounds exhibit strong Ragone effects, with rapid capacity drop-off at increased discharge rates, and appear better suited for low power applications in warm batteries. It is still very early in the development curve for LMN cathode materials, and their high energy output ensures intense industry attention. A Cr analog of this family, Li1.2Cr0.4Mn0.4O2, was produced by Pacific Lithium but failed to gain market acceptance due to potential toxicity issues.

Adding small levels of Co to LMN improves power output: perhaps other Ni replacements can do the same at lower cost. Alternately, LMN nanoparticles may exhibit improved rate capability. Since a solution (or gel) phase is required during LMN synthesis to ensure a homogeneous product, the process lends itself to spray pyrolysis (Ogihara) or related techniques used in nanotechnology. Major engineering advances are necessary to achieve commodity-level volumes (tons/week) of nano-LMN, however.

Partially lithiated MnO2 (approximately Li0.35MnO2) was advanced by Tadiran and Argonne National Labs without commercial success. The primary issue is the relatively large amount of Mn+3 in the cathode. As with spinel, this ion is very susceptible to H+-assisted disproportionation, causing dramatic capacity fade. The charge voltage maximizes at 3.5V, low enough for polymer electrolytes, however, and initial discharge capacity is at least 200mAh/g, which should insure continued attention to this material. Performance is suitable for high energy applications with low cycle number requirements, such as telecommunication backup. Raw materials are cheap and toxicity is low.

Vanadium oxides, including V2O5, V6O13, and Li1+xV3O8 (LVO), have even greater capacities and similar operating lives as Li0.35MnO2. The complexity of the crystal structures and their degradation during Li insertion and de-intercalation causes relatively rapid fade. The binary oxides are not serious contenders in commercial batteries, although LVO has its proponents (Pistoia, more recently Guyomard and 3M). LVO is acid-sensitive, but with an average potential of 2.6V it is compatible with PEO-based electrolytes. Possible uses include shallow discharge pulse and backup batteries; vanadium has vacillating pricing (currently high) and its compounds are toxic, especially as powders.

Electrolytes

Liquid electrolytes come in many guises: solutions (typically with alkyl carbonates), Bellcore-type (the solution is held by an inert ‘sponge’), and gels (a high-viscosity mix of polymer, solvent, and salt). Solvent-less gel (Angell), where the polymer dissolves in the salt, is another example. In general, these solutions contain LiPF6, which has enjoyed over a decade of success in Li-ion batteries. As the industry pushes toward less expensive cathode materials, especially those containing Mn, LiPF6 becomes less appealing. Alkylated LiPF6 derivatives can be synthesized acid-free, but solution viscosities and molecular weights are high, reducing conductivity. An alternative, Li bis(oxalatoborate) (LiBoB), holds more promise (Amine), but salt purity is a necessity that has not been consistently attained. Finally, dissolved imide salts (i.e., the triflate family) act aggressively toward battery casing metals and current collectors, but are quite stable electrochemically. LiPF6 still exhibits the highest conductivity of any of these salts in solution, and all of them either hydrolyze or are hygroscopic.

Conductive polymers are the Holy Grail for large, safe Li-ion batteries. Thermodynamics, however, dictates that as polymer conductivity improves, electrochemical stability declines. PEO-based polymers have some application, but are limited by a 3.5V degradation threshold. More promising are the N-containing polymers: polyimines (Frech and Glatzhofer) and polyimides (SoliCore). Not only is the nitrogen linkage stronger than oxygen, but also there is more opportunity for tailoring the polymer properties by affixing useful substituents to the extra N bonding site. For example, adding an ether group or the electrolyte salt anion to each N will improve Li+ transport through the electrolyte. Price is an issue for highly modified polymers, however. Lastly, Scrosati showed that adding nano-sized ceramics (Al2O3, SiO2, and others) also enhances conductivity and apparently offers some protection to the polymer against high voltage oxidation.

Which salt is most polymer-compatible? Lithium imides (Armand, 3M), with wide thermal and voltage windows, should partner nicely with polymer electrolytes as that technology matures. LiN(SO3CF3)2 (LiTFSI) offers the best combination of stability, conductivity, and molecular weight from the imide family. The usual suspect, LiPF6 , is not sufficiently robust to withstand the 50-80°C operating temperature expected for polymer electrolytes. Its analog, LiAsF6 , is very stable, but suffers the stigmata of a toxic heavy metal. Among the other common Li salts, only LiBF4 seems competitive, and it has a relatively low voltage limit (<4V). These weaknesses provide the impetus for further development.

There is an electrolyte newcomer that may solve many of the problems listed above. Room temperature molten salts, better known as ionic liquids (IL), are typically bulky organic cations paired with smaller inorganic anions, have no vapor pressure, and decompose above 300°C. Since ILs do not support combustion and are generally non-reactive, they are likely the safest electrolytes. Further, these salts frequently exhibit voltage stability above 5V, although reductive stability may be problematic. Viscosities, and therefore conductivities, of ILs vary widely with the length and atomic volume of the alkyl substituents. In general, the more asymmetric cations with moderate molecular weights provide better properties.

There are two major families of ILs, containing either imidazolium or quaternary ammonium (or phosphonium) cations. Imidazolium salts (Koch and Carlin) are very aggressive toward lithium, dissolving the intercalated metal out of graphite, and thus are unsuitable for Li-ion and lithium-metal batteries. The N-based quaternary ILs (MacFarlane) represent a work in progress (phosphonium ILs tend to very high viscosity). Potential limits extend from below 0V (vs. Li) to above 5V for selected compounds with tetra-alkyl ammonium+N(SO3CF3)2- pairings. Other anions remain to be evaluated. Conductivities of 0.5-1.0M IL/Li salt solutions are in the 10-4-10-5S range, improving above room temperature. Passerini recently suggested a gelled version with added polymer for dimensional stability. This branch of IL technology is in its infancy, meaning few sources and high prices; toxicity and environmental impact are reckoned as moderate, at worst.

Anode Materials

There are two major categories of anode materials for Li rechargeable batteries: metals (alloys) and intercalants. Lithium and Li alloys provide the highest possible energy density, but repeated cycling produces highly reactive dendrites and the possibility of fire or explosion. Dendrite growth may be reduced by judicious choice of alloys (Kehja) or polymer electrolytes; most manufacturers will not assume the risk. Tin and silicon, even mixed with other elements, reversibly alloy with Li, but also have dramatic volume increases that require compensation in the cell design. Reducing the particle size ameliorates this problem while raising a formidable economic barrier. Yazami suggested 10nm Si may be the best anode material, but it is available only in gram quantities.

Intercalation anodes are divided into two classes: graphite and analogs, such as metal borides or nitrides (cermets), and metal oxides, including tungstates, titanates, and lithiated species. The unlithiated binary or ternary compounds exhibit a common weakness: during the first charge, their crystal structures are modified by the irreversible retention of varying amounts of cathodic Li. This capture causes up to 50% capacity loss during the first charge-discharge cycle, although subsequent cycles may show acceptable capacity retention. A possible development avenue is nanomaterials, thus restricting Li+ to surface absorption and minimizing the difficulties arising from structural rearrangement.

Several LiMOx species (M is Ti, W, Mo, or multiple metals) have been suggested as anode candidates. The primary objection to these compounds is that the relatively high reduction potential (1.2-1.6V) seriously lowers the cell’s energy output. One example enjoying considerable attention is nanoparticulate Li4Ti5O12 (Amatucci, Altair Technology), which has a small hysteresis with excellent stability and rate capability, but a 1.5V anode potential. Other less-heralded materials are inverse spinels (LiMVO4 where M is divalent), which intercalate Li below 1V, the preferred range (Reddy). First-cycle capacity hysteresis with these and other LiMOx compounds present a performance hurdle, however.

Conclusions

Tables 1-3 provide a semi-quantitative summary for the properties of current and possible battery materials for lithium rechargeables. Readers are encouraged to review publications by the scientists and companies mentioned in the text for more details.

End users are demanding more power, greater energy, and longer life from safe, environmentally friendly rechargeable batteries. Based on known chemistries, the following are sure to be on someone’s stored power development list.

1. Layered LiMnNi oxides, providing >1Wh/g energy. These cathodes will be coupled with

2. High voltage electrolytes, such as polyimines or quaternary ammonium ionic liquids incorporating

3. Triflate-related salts, most likely LiN(SO2CF3)2 replacing LiPF6 .

4. Inexpensive Li manganese oxide spinels with rapid discharge capability, matched with an electrolyte system from 2 and 3, targeting EV/HEV.

5. Highly stable LiFePO4 and other phospho-olivines (already well advanced) with almost any electrolyte.

6. Li+-intercalating nanomaterials, especially for anodes, to enhance pulse operation and minimize electrode volume changes. This will require engineering breakthroughs to push production to multi-ton levels, else these species will be uneconomical as commodity chemicals.

The creativity of global battery scientists and engineers ensures that novel materials and techniques will replace today’s technologies. Indeed, parts of the above list may be already obsolete: therein lies the challenge – and the enjoyment – of product development.

About the Author

W. F. (Rick) Howard has nearly 15 years experience with lithium battery materials development, most recently as senior manager of Kerr-McGee Stored Power R&D, and is currently a chemical and R&D management consultant. Dr. Howard has more than 30 publications and nine patents (four pending). Contact rikhoward@aol.com.