(Keynote) Comparison of the Electrode Materials for Li and Na Ion Batteries
Authors: Huang Xuejie , Hu Yongsheng 
Published: February 2020

Secondary battery is becoming the key enabling technology for 3C products, electric vehicles (EVs) and energy storage system (ESS). Lithium-ion batteries (LIBs) with high energy density are widely used for mobile electronics, EVs and energy storage. Due to the abundant of resources, sodium-ion batteries (SIBs) have been considered as the promising candidate for large-scale energy storage system owing to the economic efficiency.

LIBs and SIBs share similar concept and many materials. This presentation will summarize and compare their anode and cathode materials. Carbonaceous materials are commonly used for both of them as anode materials. However, well crystallized graphite is a good anode material for LIBs but not for SIBs. We have made hard carbon from sugar and anthracite for Li+ and Na+ storage. Sn, Sb, Si etc., which can alloy with Li and Na, show higher Na storage capacity. The rate capability of battery electrodes is highly dependent on the grain size, texture, and morphology of the electrode materials. An ordered, high surface area structure of electrochemically active materials on the nano scale can yield enhanced charge/discharge characteristics. Using nano materials as electrodes presents new opportunities for energy density, exceptionally high rate of charge and discharge, high cyclability, and low cost. The spinel Li4Ti5O12, well-known as a ‘zero-strain’ anode for lithium-ion batteries, can also store sodium, displaying an average storage voltage of 0.91 V and a reversible capacity of 155 mAh g1 and presents the best cyclability among all reported oxide-based anode materials. A three-phase separation mechanism, 2Li4Ti5O12 + 6Na+ + 6e- → 2Li7Ti5O12 + Na6LiTi5O12, has been confirmed. Furthermore, we also proposed many organic anode materials for SIBs including: Na2C8H4O4, Na2C6H2O4, Na2C14H6O4. In particular, we resolved the crystal structure of Na2C6H2O4 and found that it consists of alternating Na-O octahedral inorganic layer and π-stacked benzene organic layer in spatial separation. The experiment and calculation results reveal the Na-O inorganic layer provides both Na+ ion transport pathway and storage site, while the benzene organic layer provides electron transport pathway and redox centre.

A large variety of cathode materials have been proposed based on the abundant knowledge developed on LIBs. They include layer- and spinel-structured transition metal oxides, and poly-anionic-type compounds. While layer-structured oxides with Co show high capacity for LIBs, the poly-anionic-compounds，spinel-type oxides and organic cathode materials are more promising for wide range of applications. Spinel-type oxides are used for LIBs only and certain nano-layered materials protected their surface and improve their cycling performance even at elevated-temperatures. High-voltage spinel LiNi0.5Mn1.5O4 cathode material has specific energy (~640 Wh/kg) due to the high operation voltage of ~4.7 V. The relatively inexpensive Ni and Mn in LiNi0.5Mn1.5O4 make this cathode material particularly desirable for large-scale applications. The surface regions (~ 2nm) show an irreversible migration of TM ions into lithium tetrahedral sites to form a Mn3O4-like structure. When the surface LiNi0.5Mn1.5O4 was modified by coating and superficial doping, the cycling performance of the material at elevated temperatures has been much improved and the Coulombic efficiency of discharge/charge is also obviously increased. Theoretical calculations analyses reveal that the stability of O at surface layer is enhanced by those surface modifications.

Many layered oxide cathode materials with a general formula of NaxMO2 (M = Ni, Co, Mn, Fe, Cr, etc.) have attracted great attention for SIBs. We discovered that a novel Cu2+/Cu3+ redox couple is electroactive in Na containing layered oxides. On the basis of this important finding, we designed a new layered oxide, O3-Na0.9[Cu0.22Fe0.30Mn0.48]O2, showing unexpectedly superior stability against moisture and excellent electrochemical cycling stability. The material delivers a reversible capacity of 100 mAh/g with an average voltage of 3.2 V and stable cycling. A prototype sodium-ion pouch cell using our Cu based cathode and anthracite derived carbon anode is demonstrated to have an energy density of 117 Wh/kg and good cycling as well as high safety.

The perspective of future practical application will be discussed based the comparison of materials.