Lithium batteries supplied with cellular telephones and other personal electronic devices provide unprecedented power and capacities in very small formats. They are able to deliver such high performance because they incorporate highly reactive materials in both the positive and negative electrodes, resulting in individual cell potentials of nearly 4 V. Exposure to high temperatures or abusive treatment including overcharging can cause catastrophic failure of these batteries, resulting in gas venting, fire, or even explosion. Mechanical and electronic safety devices are employed to prevent the development of dangerous conditions. While these are generally effective, they add significantly to the weight, volume and cost of lithium battery packs. In high voltage packs for power tools and traction motors, some of these devices become unworkable or prohibitively expensive. An internal chemical mechanism for overcharge protection could replace or at least augment external safety devices. A number of electrolyte additives that can extend the useful lifetime of moderately overcharged cells and/or prevent hazardous reactions in severely overcharged cells have been discovered. Development of abuse-tolerant electrode materials that reduce the potential for hazardous events will also be described.