PROGRESS REPORT © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim (1 of 17) 1600906 wileyonlinelibrary.com Lithium- and Manganese-Rich Oxide Cathode Materials for High-Energy Lithium Ion Batteries Jun Wang, Xin He, Elie Paillard, Nina Laszczynski, Jie Li,* and Stefano Passerini* DOI: 10.1002/aenm.201600906 1. Introduction Ever-growing population and depletion of fossil fuels within recent years have triggered large demand for alternative energy production, storage and distribution systems. In the field of energy storage, rechargeable/secondary batteries are the most energy efficient storage devices that convert off-peak electricity into chemical energy and release the stored energy reversely during on-peak periods. Today, secondary battery technologies mainly include lead acid, nickel-metal hydride (Ni-MH), lith- ium-ion and redox flow cells. [1] Amongst them, lithium-ion bat- teries (LIBs) have occupied a dominant position in consumer Layered lithium- and manganese-rich oxides (LMROs), described as xLi 2 MnO 3 •(1–x)LiMO 2 or Li 1+y M 1–y O 2 (M = Mn, Ni, Co, etc., 0 < x <1, 0 < y ≤ 0.33), have attracted much attention as cathode materials for lithium ion bat- teries in recent years. They exhibit very promising capacities, up to above 300 mA h g -1 , due to transition metal redox reactions and unconventional oxygen anion redox reaction. However, they suffer from structural degrada- tion and severe voltage fade (i.e., decreasing energy storage) upon cycling, which are plaguing their practical application. Thus, this review will aim to describe the pristine structure, high-capacity mechanisms and structure evo- lutions of LMROs. Also, recent progress associated with understanding and mitigating the voltage decay of LMROs will be discussed. Several approaches to solve this problem, such as adjusting cycling voltage window and chemical composition, optimizing synthesis strategy, controlling morphology, doping, surface modification, constructing core-shell and layered-spinel hetero struc- tures, are described in detail. Dr. J. Wang, Dr. J. Li MEET Battery Research Center University of Muenster Corrensstrasse 46, 48149 Muenster, Germany E-mail: jie.li@uni-muenster.de X. He, Dr. E. Paillard Helmholtz-Institute Muenster (IEK 12) Forschungszentrum Juelich GmbH Corrensstrasse 46, 48149 Muenster, Germany N. Laszczynski, Prof. S. Passerini Helmholtz-Institute Ulm (HIU) Karlsruher Institute of Technology (KIT) Helmholtz Strasse 11, 89081 Ulm, Germany E-mail: stefano.passerini@kit.edu electronic devices due to their high energy densities (both volumetric and gravi- metric). LIBs are presently considered as energy storage devices to power (hybrid) electric vehicles (HEVs and EVs) and bal- ance the supply-demand of renewable energy plants. [2] Currently conventional LIBs differ only little from the first LIB developed by Sony in the early 1990s, which com- prised a carbonaceous negative electrode, a lithium cobalt oxide (LiCoO 2 ) positive electrode and an organic carbonate-based solution containing a lithium salt as electrolyte. [2e] It is well known that positive electrode materials (also called cathode materials) constitute the bottleneck for present LIBs. [3] State-of-the-art positive electrodes are based on layered struc- tures (LiCoO 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 and LiNi 0.33 Co 0.33 Mn 0.33 O 2 ), spinel (LiMn 2 O 4 ) and olivine LiFePO 4 , with rather moderate specific capaci- ties (100–180 mA h g -1 ). However, as shown in Figure 1, the resulting specific energies are insufficient for enabling, for instance, electric vehicles with long driving range (500 km) but a reasonably sized and priced battery pack. Hence, cathode materials with higher energy (i.e., capacity × voltage) are of primary concern. During the past two decades, many efforts on exploiting new cathode materials have been made. [3,4] Amongst the reported cathode materials so far, layered lithium- and manganese-rich oxides (LMROs) have attracted significant attention in recent years. [5] LMROs can deliver high specific capacities (>300 mA h g -1 ) with an average discharge voltage of >3.5 V, resulting, when paired with advanced nega- tive electrodes, in battery pack energy densities approaching 1000 Wh L -1 (Figure 1). [5j,6] In addition, LMROs are economi- cally attractive due to their high content of manganese, which is much cheaper and less toxic than cobalt. [5j,7] However, despite their high capacities, LMROs have several pitfalls: (1) large irre- versible capacity loss in the first cycle attributable to the release of oxygen and Li from its lattice toward the end of the first charge; [7d,8] (2) poor rate capability related to low electronic con- ductivity because of the Mn 4+ ions and thick SEI layer formed by the reaction of the cathode surface with the electrolyte; [9] (3) insufficient cycling performance under high cut-off charge volt- ages; [10] and (4) gradual voltage decay during cycling process. [11] In particular, the voltage decay is not yet fully understood and remains the greatest challenge for the practical use of LMROs since it leads to a continuous energy density loss during cycling Adv. Energy Mater. 2016, 1600906 www.MaterialsViews.com www.advenergymat.de