© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 922 REVIEW wileyonlinelibrary.com www.MaterialsViews.com www.advenergymat.de DOI: 10.1002/aenm.201200068 Dr. A. Kraytsberg, Prof. Y. Ein-Eli Department of Materials Engineering Technion-Israel Institute of Technology Haifa 32000, Israel E-mail: eineli@tx.technion.ac.il Alexander Kraytsberg and Yair Ein-Eli* Higher, Stronger, Better … A Review of 5 Volt Cathode Materials for Advanced Lithium-Ion Batteries The ever-increasing demand for high-performing, economical, and safe power storage for portable electronics and electric vehicles stimulates R&D in the field of chemical power sources. In the past two decades, lithium-ion technology has proven itself a most robust technology, which delivers high energy and power capabilities. At the same time, current technology requires that the energy and power capabilities of Li-ion batteries be ‘beefed up’ beyond the existing state of the art. Increasing the battery voltage is one of the ways to improve battery energy density; in Li-ion cells, the objective of current research is to develop a 5-volt cell, and at the same time to maintain high specific charge capacity, excellent cycling, and safety. Since current anode materials possess working potentials fairly close to the potential of a lithium metal, the focus is on the development of cathode materials. This work reviews and analyzes the current state of the art, achievements, and challenges in the field of high-voltage cathode materials for Li-ion cells. Some suggestions regarding possible approaches for future development in the field are also presented. inside the cathode material, being oxi- dized by a transition metal redox couple. Whereas lithium mobility in the carbon anode is sufficiently high, the development of cathode materials with substantial Li + - mobility turned out to be an issue of prime importance. Such a material was first pre- sented by Whittingham, [1] who employed a TiS 2 -based cathode material in a cell with a metallic Li anode. The structure of TiS 2 comprises layers of hexagonal close- packed octahedral atomic groups, formed by a layer of titanium atoms between two layers of sulfur atoms, [5] thus allowing insertion of Li + into the layered gap. Upon discharge, Li + ions occupy the vacant octa- hedral sites between the layers; the charge balance is maintained by electron current via the external circuit, converting Ti 4 + into Ti 3 + . A reverse process occurs on charging, maintaining the pristine TiS 2 structure. This work promoted research on other sulfides and chalcogenides during the 1970s and 1980s; how- ever, cells employing such cathodes exhibited insufficient volt- ages of V cell < 2.5 V. In the beginning of the 1980s Goodenough et al. started working with oxide cathode material LiCoO 2 ; this layered oxide, having the structure similar to the structure of LiTiS 2 , demonstrates V cell > 4 V. [2] The approach paved the way to safe Li-ion cells but required the development of practical anode/cathode materials, which remain undamaged over numerous Li + insertion/extraction cycles; also, the development of adequate nonaqueous electro- lytes [6,7] was needed. In the early 1990s, Sony succeeded in the commercialization of the first rechargeable Li-ion cell based on a carbon anode (petroleum coke) and a LiCoO 2 cathode; the cell demonstrated an open circuit voltage of over 3.6 V and an energy density of ∼150 Wh kg -1 . [8,9] Since then, Li-ion bat- teries have been recognized as high energy and high operation voltage, rechargeable power sources, [10] outperforming other available battery systems in terms of energy density, design flexibility, cycle life, and low self-discharge rate. These features make them the ideal choice for mobile electronic devices and also an appealing option for hybrid and electric vehicle energy storage. Current R&D in this field is focused on developing high voltage cathode materials with a high charge capacity and cycling capability; substantial efforts are also being applied towards the development of organic electrolytes with a broad voltage window and high conductivity. In addition, the issues of 1. Introduction The most essential parameters in chemical energy storage devices (batteries) are specific energy, energy density (in both cases, the larger the better), cost (the lower the better), and safety. The cell specific energy and energy density depend, first of all, on the cell chemistry, being reflected in its potential and charge capacity values. From this standpoint, Li-based cells hold much promise because Li metal is the most electroposi- tive (E 0 = -3.04 V vs. standard hydrogen electrode) and light ( ρ = 0.53 g cm -3 ) material. However, employing Li metal in a secondary cell is challenging, since the possibility of dendrite growth poses risks of anode-cathode shorting (followed by the instant release of all stored energy). In the 1970s–1980s, the concept of a Li-ion cell (“rocking chair battery”) was demonstrated; [1–4] this concept was based on the substitution of a Li metal anode with Li-ion intercala- tion compounds. The lithium is in an “almost atomic” state in a carbonaceous anode material, and it is in an “almost Li + ”-state Adv. Energy Mater. 2012, 2, 922–939