STUDIES ON SPINEL MANGANITES AS PROSPECTIVE LITHIUM ION BATTERY ANODES

Abstract

Electrode materials are the important constituents of any battery systems, and the identification of viable anodes and cathodes materials is critical. Present state-of-the-art Lithium ion batteries utilize mainly graphite as the anode material owing to its low cost, longer cycle life. However, it exhibits comparatively low experimental capacity i.e. ~330 mAh g-1 at almost zero potential vs. Li/Li+. Further, low potential intercalation at high rate causes the deposition of Li on graphite leading to dendrite formation. This continuous deposition of Li-ion over surface of anode results the short circuiting of Li-ion batteries (LIBs) and causes thermal runaway in LIBs. In regard to this, continuous efforts are being made to invent new alternative anode material which could fulfill the requirement of futuristic battery technology (low cost, high energy and power density, and good longevity) useful for high end application viz. space, defence, stationary power back up etc. In order to replace graphite in commercial LIBs, for the last two decade, based on energy storage mechanism, a number of materials i.e. intercalation based materials (graphite, TiO2, Li4Ti5O12, etc.), alloying-de-alloying based materials (Sn, Si, etc), and conversion based oxides (CoO, FeO, NiO, etc) have been studied as prospective LIBs anodes. Referring the ongoing research trend to choose an alternative to graphite, spinel AMn2O4 is considered to be a special class material for LIBs anode. Choice of counter cation ‘A’ plays an important role to tune the average charge potential (vs Li/Li+), capacity, cyclability of material used as LIB anode. Considering the literatures studied in the recent past of prospective anode material for LIB anode, special attention is being paid to develop a new class of material which undergoes “conversion as well as alloying/de-alloying reactions”, and aids each other. Alloying based element/intermetallic may provide ionic/electronic conducting channels inside oxides framework through which the participation of active material present in electrode may be increased and thereby high capacity is achieved in the past. In view of above, Zn, Sn, Si etc. may be opted as an element as A in AMn2O4. However, depending upon the ionic radius, oxidation state, few of above material can form spinel structure with Mn-based oxides. Based on these facts, in the literature, ZnMn2O4 was prepared and studied as LIB anode and high capacity (~ 800 mAh g-1) was obtained. However, poor-capacity retention and -rate capability were major problem even if Zn undergoes both the reactions called “conversion and alloy-dealloy”. However, in order to find suitable counter cation ‘A’, cadmium, due to its ability to make alloy with Li (Li3Cd) and therefore consuming total 5 Li, was chosen and surprisingly, highly stable capacity, rate capability was obtained. In addition to this, because cadmium exhibits high wear resistance, good ductility, better mechanical and fatigue strength therefore, it may be considered as good alternative choice for alloying-de-alloying matrix (in comparison to Zn). Particularly, a reversible stable capacity of 610 (±10) mAh g-1 is obtained when cycled in the range, 0.005-3.0 V vs. Li at a current rate of 200 mA g-1 for CDMO. However, ZMO exhibited capacity fading. The present study showed the importance of high ductile and wear resistance alloying material, Cd to stabilize the cyclability of Mn-based oxides. It is further showed that an optimum size of nanoparticles perform much better than nanofiber by alleviating the induced stress during cycling. Further, fundamental insight on Li-migration in these systems in terms of structural transformation and their correlation with capacity retention has been studied. Furthermore, it is also observed that Cd-based manganites exhibit lesser volume variation in unit cell in comparison to Zn-based manganites. Moreover, structural integrity and stability in initial cycles do impact on cycling properties of LIBs. Though, Cd is found to be a superior than Zn, but it is toxic; which need to be taken care of. Hence, small amount of Cd in purposefully incorporated in Zn-Mn framework, and corresponding Li-storage properties are studied. All the doped samples exhibited improved capacity retention till 100th cycles with the values ~ 1100 mAh g-1 and 765 mAh g-1 for Zn0.9Cd0.1Mn2O4 and Zn0.8Cd0.2Mn2O4, respectively. This improved cycling which further justify that mechanical properties of alloying elements are important to buffer the volume variation during cycling. And in this case, Cd have shown its appropriateness (even if it is 10%) for stable and high capacity.