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The structures of these LLOs are still currently being debated (yellow region in Figure 12). The local structures are very important to the electrochemical performance, especially the rate performance of these LLOs. After synthesizing a lot of these LLOs, we found that the electrochemical properties of these LLOs were extremely sensitive to the preparation conditions and composition change, although their XRD or SXRD patterns are very similar. These phenomena may be contributed to the imperceptible variation of local structures, because the local environments govern properties such as the activation barriers, strain fi elds, and steric hindrances of the electrode materials, and can a ff ect the lithium ion transport inside the electrode materials by blocking or opening the lithium ion pathways. The investigation with some novel analysis techniques, especially in situ testing methods (in situ TEM, Raman, neutron, etc.), on these LLOs will help us to reveal the more detailed nature of these LLOs, understand the reason for their sensitive electrochemical performance, and fi nd the relationship between local structure and electrochemical properties.

It is very useful to understand the reaction mechanism (blue region in Figure 12) of these materials at room temperature by the three-dimensional phase diagram. However, it is still di ffi cult to understand the large mysterious abundant capacity of these LLOs at high temperature. The reaction mechanism of these LLOs at high temperature may be very di ff erent from that at room temperature. Some redox reactions associated with lithium ion extraction/insertion from/into some special sites may need to hurdle a larger energy barrier. The reaction mechanism of these LLOs at di ff erent environment temperatures should be focused on in the future.

(3) The electrochemical properties associated with low initial columbic e ffi ciency, slow Li + di ff usion speed, and voltage degradation during cycling of these LLOs are the main problems (green region in Figure 12) preventing their utilization in lithium-ion batteries. Although the surface coating with oxides, fl uoride, and other resistant materials can lower the surface reactivity, provide a robust surface, and e ff ectively improve the initial columbic e ffi ciency, cycle stability, and rate performance of these LLOs, it cannot absolutely prevent their internal phase transformation. Thus, how to minimize the internal structure change during cycling and balance the structure variation and electrochemical performance need to be researched. Doping with some elements into the internal structure of these LLOs may be help to improve the structure stability.

In general, although there are many debates on these LLOs, they are still attractive for utilization as cathode materials in lithium ion batteries because of their large rechargeable capacities, thus, much e ff ort with international collaboration needs to be conducted on these LLOs in the future.

The authors declare no competing fi nancial interest.

Haijun Yu received his Ph.D. (2007) in Metallurgy Science and Engineering from the Northeastern University. In 2007-10, he worked as senior engineer at the General Research Institute for Nonferrous Metals (GRINM) in China on batteries and battery-related materials research. He is currently researching electrode materials and novel batteries for the next-generation energy storage system at the National Institute of Advanced Industrial Science and Technology (AIST), Japan.

Haoshen Zhou is now a prime senior researcher of the Energy Technology Research Institute (ETRI), National Institute of Advanced Industrial Science and Technology (AIST), and leading the Energy Interface Technology Group, ETRI, AIST, Japan. He is a guest invited professor at both The University of Tokyo and Nanjing University. His research interests include the synthesis of functional materials and their applications in lithium ion batteries, metal-air batteries, new type batteries/cells. Web page: htm