Polymorphic Consideration in Conversion Reaction-Based Anode Materials for Next-Generation Li Rechargeable Batteries
Authors: Kim Hyunwoo , Yoon Jaesang , Yoon Won-Sub 
Published: May 2020

Beginning with the commercialization by Sony in the 1990s, Li-ion batteries (LIBs) have been used in many electronic devices. In commercialized LIBs, insertion reaction-based electrode materials, such as LiCoO2, LiFePO4, LiMn2O4 for the cathode and graphite, Li4Ti5O12, TiO2 for the anode, have been dominantly used as active materials. In these materials, Li-ions can reversibly be stored and extracted at the specific interstitial site in the crystal without severely collapsing the initial crystal structure. As so, the crystallographic phase significantly affects the electrochemical performances because the local environments around the interstitial site are all different among these phases. For example, LiFePO4 has stable FePO4 framework even after most Li-ions have been extracted, although some Li-ions confined in the tunnel block the migration paths due to the one-dimensional tunnel inside the crystal. In contrast, Li-ions can move easier in LiCoO2 when comparing with LiFePO4 even if the Li-ions are trapped in the Li slab, but over lithiation results in the collapse of the layered structure leading to capacity fading. Even in the same composition, the electrochemical Li storage behavior varies much by the polymorphs. For instance, the layered LiCoO2 exhibit larger reversible capacity and higher redox potential compared to spinel structured Li2Co2O4 [1], and in case of TiO2, despite the similar amount of Li-ion storage, in detail, the voltage profiles are different in anatase, rutile and brookite TiO2 [2].
Since the demand for high capacity energy storage devices is continuously growing, there have been extensive researches on high energy density electrode materials based on alloying and conversion reaction due to the limited capacity of insertion-based electrode materials. Manganese dioxide (MnO2) has been considered as a potential candidate due to its low cost, environmentally benignity and especially large theoretical capacity of ~1233 mAh g-1. Accordingly, various types of polymorphs in MnO2 (α, β, γ, δ, ε, λ, etc.) have been adopted as the anode materials in LIBs. However, unlike the insertion-based electrode, the effect of crystal structure in electrochemical conversion reaction was note clearly identified because it is hard to identify the detailed phase transition process as the initial crystalline structures transformed to the amorphous state after lithiation. Recently, some researchers compared the Li storage performance of different polymorphs of MnO2 [3,4], but the morphology of the materials are quite different or the reversible capacities are far below the theoretical value based on the conversion reaction, which could not sufficiently explain the polymorphic effects in conversion reaction.
In this work, we have prepared 4 different polymorphs of MnO2 (α, β, Ramsdellite, and λ) in similar hollow urchin-like morphology and compared the Li storage behaviors to discover the influence of polymorphs in conversion-based electrode materials. Among those 4 polymorphs, λ-MnO2 exhibits the highest reversible capacity of ~1200 mAh g-1 and the fastest reaction kinetics. To demonstrate the structural advantages of λ-MnO2, we observed structural changes during electrochemical reactions using synchrotron-based X-ray absorption spectroscopy and X-ray crystallography. As a result, we discovered that the overall Li storage behaviors in the 4 different polymorphs are nearly the same, however, the structural similarity between the initial polymorphs and the final polymorph after reversible conversion reaction affects the electrochemical performance. More detailed discussion will be presented at the time of meeting.
References
[1] S.G. Kang et al., Solid State Ionics, 120 (1999) 155–161
[2] D. Dambournet et al., Chem. Mater, 22 (2010) 1173–1179
[3] N. Kumar et al., RSC Adv, 7 (2017) 25041–25053
[4] K. Chen et al., J. Phys. Chem., 117 (2013) 10770–10779