Development of High-Capacity Intercalation Compounds for Lithium-Ion Batteries

Intercalation compounds have been the most important class of energy storage materials for both Li-ion and alkaline rechargeable batteries, and various aspects of the material have been improved throughout the past decade to make batteries with better performance. One of the pressing issues in Li-ion batteries is to further increase their capacities, despite that the improvement of current intercalation materials appears to be slowing down. Much research has been devoted on the development of new cathode intercalation compounds with higher capacity as an alternative and a few of the strategies will be discussed in this presentation. One strategy to increase material capacity is to increase the number of reversible lithium ions. Overlithiated transition-metal oxide materials have been gaining attention [1-5] because their capacities may be increased by placing Li in the transition-metal sites. For example, Li1.20Mn0.54Ni0.13Co0.13O2 (often written as 0.6Li(Li1/3Mn2/3)O2 + 0.4LiMn1/3Co1/3Ni1/3O2) shows a discharge capacity of 270 mAh/g between 2.0V and 4.8V vs. Li/Li. Volumetric energy density of the cathode material is 4060 Wh/L (calculated with Li as the anode), 30% higher than that of LiCoO2 charged to 4.3 V vs. Li/Li and still higher than that of LiCoO2 even if it is charged up to 4.5 V vs. Li/Li. For typical cathode material such as LiCoO2 with a rock-salt structure (R3-m), structural instability at high voltages limits the number of lithium ions that can be inserted and extracted reversibly. Higher capacity can thus be obtained if there is a method to stabilize the host structure with transition metal and oxygen atoms. For example, LiMO2 (M: transition metals) with a layered structure different from R3-m can be obtained by initially synthesizing the complementary material with sodium and then exchanging the sodium ions with lithium ions [6-8]. This ion-exchange technique may stabilize the material structure to enable more lithium ions to be extracted at a higher voltage. In our recent experiments, the material obtained from the ion-exchange of Na0.7LixMn0.5Co0.5O2 reveals an O2 structure that gives a discharge capacity over 200mAh/g based on the intercalation/deintercalation of nearly 1 mole lithium ion per unit between 2.5V and 5.0V vs. Li/Li. In spite of their good potential as high-capacity cathode materials, there are still problems that need to be solved for practical applications. Possible solutions and new challenges will be addressed in the presentation.