Capacity Cathode Material Research Articles

Thin-Film SiOx Coated Carbon Nanofibers As Stable Anodes for Lithium-Ion Batteries

With the increase in the capacity of cathode materials in lithium-ion batteries(LIBs), the demand for novel anode materials with high theoretical energy density is following. Up to now, many candidates have been reported; silicon-based composites are considered a prospective alternative due to their abundant reserves and high energy density. However, The primary disadvantage of Si, electrochemical irreversibility during the charge/discharge process, hinder widespread use.In this study, we adapted SiOx-coated carbon nanofibers (SiOx-CNFs) composite to the binder-free anode current collector for lithium-ion batteries. The composite has a structure where a thin amorphous SiOx (thickness ~100nm) is coated on the CNFs surface and is successfully synthesized by electrospinning and sputtering. This structure, surface SiOx-coated porous CNF, breaks through the significant drawback of Si-based composites, pulverization of Si element during the charge/discharge cycle. In addition to this, due to the structure of porous CNF, we achieved the curtail of electrode weight.Scanning Electron Microscopy (SEM) and X-ray diffraction (XRD) analyses are carried out to confirm the morphology and crystallinity. Furthermore, with cyclic voltammetry analysis, fast reaction kinetics and excellent electrochemical reversibility are verified. The cycle test is also conducted to prove excellent capacity and cyclic stability.

Effects of Li+ conduction on the capacity utilization of cathodes in all-solid-state lithium batteries.

Li+ conduction in all-solid-state lithium batteries is limited compared with that in lithium-ion batteries based on liquid electrolytes because of the lack of an infiltrative network for Li+ transportation. Especially for the cathode, the practically available capacity is constrained due to the limited Li+ diffusivity. In this study, all-solid-state thin-film lithium batteries based on LiCoO2 thin films with varying thicknesses were fabricated and tested. To guide the cathode material development and cell design of all-solid-state lithium batteries, a one-dimensional model was utilized to explore the characteristic size for a cathode with varying Li+ diffusivity that would not constrain the available capacity. The results indicated that the available capacity of cathode materials was only 65.6% of the expected value when the area capacity was as high as 1.2 mAh/cm2. The uneven Li distribution in cathode thin films owing to the restricted Li+ diffusivity was revealed. The characteristic size for a cathode with varying Li+ diffusivity that would not constrain the available capacity was explored to guide the cathode material development and cell design of all-solid-state lithium batteries.

Building a high-performance organic cathode material containing electron-withdrawing groups for lithium-ion batteries

The capacity of cathode materials is one of the main factors to limit the performance of lithium-ion batteries (LIBs), so it is urgent to develop high-performance cathode materials. Herein, trinitrohexaazatrinaphythylene (TNHATN) including an electron-withdrawing group (nitro, -NO2) was synthesized by the condensation reaction between hexaketocyclohexane and 4-nitro-o-phenylenediamine, and it was first investigated as a cathode material for lithium-ion batteries. The TNHATN electrode displays a high discharge specific capacity of 355.7 mAh g−1 at 0.05 A g−1 and a superior cycling stability, having the capacity retention of 97.9 % after 200 cycles. The excellent behaviors may be ascribed to its π-conjugated structure including electron-withdrawing groups and multiple redox active sites. The experimental results reveal the redox active sites are pyrazine nitrogen atoms and oxygen atoms from nitro groups. This work confirms that it is an effective route to introduce an electron-withdrawing group into a π-conjugated compound for obtaining high-performance organic cathode materials of LIBs.

Unlocking the Capacity of Vanadium Oxide by Atomically Thin Graphene‐Analogous V2O5·nH2O in Aqueous Zinc‐Ion Batteries

AbstractAqueous Zn‐ion batteries (AZIBs) are promising due to their high theoretical energy density and intrinsic safety, and the natural abundance of Zn. Since low voltage is an intrinsic shortage of AZIBs, achieving super‐high capacity of cathode materials is a vital way to realize high practical energy density, which however remains a huge challenge. Herein, the capacity increase of classical vanadium oxide cathode is predicted via designing atomic thickness of 2D structure to introduce abundant Zn2+ storage sites based on density functional theory (DFT) calculation; then graphene‐analogous V2O5·nH2O (GAVOH) with only few atomic layers is fabricated, realizing a record capacity of 714 mAh g−1. Pseudocapacitive effect is unveiled to mainly contribute to the super‐high capacity due to the highly exposed GAVOH external surface. In situ Raman and synchrotron X‐ray techniques unambiguously uncover the Zn2+ storage mechanism. Carbon nanotubes (CNTs) are further introduced to design GAVOH‐CNTs gel ink for large‐scale cathode fabrication. The hybrid cathode demonstrates ultra‐stable cycling and excellent rate capability and delivers a high energy density of 476 Wh kg−1 at 76 W kg−1; 228 Wh kg−1 is still retained at high mass loading of 10.2 mg cm−2. This work provides inspiration for breaking the capacity limit of cathode in AZIBs.

Innovations in Post-Mortem Battery Material Characterization for Diagnosing Failure Mechanisms

An increasing demand for clean and renewable energy provides motivation for additional innovations to be sought in energy storage. Lithium-ion batteries (LIBs) have seen many advancements in terms of their cathode chemistries, including electrode coatings that are used to increase durability and capacity of cathode materials. These coatings are effective, but create additional interfaces in the battery system, that adds to complexity in characterization. To address many key issues of LIB failures, further insights are necessary that provide details on currently unknown and partially known intermediate states, side reactions, and/or mechanisms of degradation at electrode interfaces. In this work, a method is developed to enable a detailed surface analysis and additional characterization of coated cathode particles harvested from post-mortem LIBs. Procedures are outlined for the safe handling and preparation of post-mortem materials for high-quality single particle electron microscopy analyses, paired with ultramicrotome techniques for high-throughput cross-sectional imaging with elemental and structural analyses of the coatings themselves.Commonly seen in post-mortem results, the materials are kept intact as whole electrodes or large collections of particles while still held within binders and other cathode slurry components from cell fabrication. These samples are often briefly rinsed with electrolyte solvents [e.g., most commonly dimethyl carbonate (DMC)] to rinse away residual lithium salts.1 While important insights have been gained from such studies, details of surface defects are difficult to discern amidst the debris of additives and binders. To address this, we developed a non-destructive procedure for the separation of cathodes particles from their supports, followed by washing to remove binders, additives, and residual electrolyte with relatively safe and accessible solvents. This method enables high-quality single particle imaging by scanning and transmission electron microscopy (SEM and TEM) techniques. Nano-scale features on the particle surfaces can be observed with high-resolution on the post-mortem cathode particles. With this procedure, insights can be gained about the retention of coatings after battery cycling, in addition to observing visible surface defects that result from charge cycling.Typical cross-sectional analyses of interfacial systems involve expensive, time-consuming methods like focused ion beam (FIB) milling that require extensive training, and are low-throughput when FIB lift-out samples are prepared for TEM. An alternative high-throughput technique for the cross-sectional analysis of coated cathode particles by ultramicrotome was previously developed in our group.2 This technique involves embedding coated cathode particles in epoxy, from which ultra-thin slices (<100 nm) that can contain many [approx. 50-200] particles at a time are cut with a diamond knife. The resultant sheets of epoxy are deposited onto TEM grids so the cross-sections can be imaged by TEM. These sections also enable the particle coatings to be characterized separately from the bulk cathode by electron dispersive X-ray spectroscopy (EDS) and selected area electron diffraction (SAED). This technique was demonstrated on as-synthesized (pre-electrochemistry) samples but was adapted here for the prepared post-mortem samples to assess degradation of the particles and their coatings.Combining the newly developed techniques for obtaining high-quality images of single whole-particles and particle cross-sections this work provides a safe, relatively inexpensive, and high-throughput methodology for the post-mortem characterization of LIB materials. This data can be correlated to as-synthesized materials and electrochemical cycling data to create a detailed profile of cathode aging and degradation as it relates to LIB failures. We are expanding this method to a variety of standard and coated cathode systems to provide detailed information needed for the next steps towards developing new innovations in electrode designs aimed at achieving more durable LIBs.

Review of synthesis and structural optimization of LiNi1/3Co1/3Mn1/3O2 cathode materials for lithium-ion batteries applications

Abstract Lithium-ion batteries (LIBs) have garnered significant academic and industrial focus because of their excellent merits, like high voltage, high energy density, excellent cyclic performance, no memory effect and environment-friendly nature. So far, the electrochemical properties of LIBs are restricted by the capacity of cathode materials and various novel compositions and designed architectures have been proposed to enhance the energy density and cyclic performance of LIBs cathodes. Recently, lithium nickel cobalt manganese oxides have attracted extensive research interest owing to the united advantages of LiCoO2, LiNiO2 and LiMnO2. Herein, we have reviewed the recent developments of LiNi1/3Co1/3Mn1/3O2 from the viewpoint of synthesis processes and structural designs. The electrochemical properties of LiNi1/3Co1/3Mn1/3O2 depend on particle size, morphology, ion doping and surface coating of the as-prepared cathode powder. The present article summarizes the recent developments and provides an insight into the future roadmap for the realization of high energy density LIBs.

Unveiling ionic diffusion in MgNiMnO4 cathode material for Mg-ion batteries via combined computational and experimental studies

A major challenge in the field of rechargeable Mg batteries is the development of high voltage/high capacity cathode materials. Naturally, a first step in a general search of cathode materials for Mg batteries should be following the plethora of cathode materials relevant to Li-ion batteries. Indeed, several compounds that were thoroughly studied in connection to Li-ion batteries were found to interact reversibly with Mg ions, as well. The functionality of metal ion batteries relies on an efficient ionic transport within the electrodes’ active mass. In this study, we examined the extreme case of the MgNiMnO4 material, using a combination of computational and experimental techniques. The scientific question being raised in this study was whether Mg ions can be extracted electrochemically from this compound. The experiments provided a negative answer and calculations based on density functional theory (DFT) + U showed that indeed Mg ions diffusion in this material is energetically unfavorable. It was confirmed again how computational work can be very useful in predicting barriers for ionic diffusion in hosts and hence, can save much of tedious experimental works.

LiFePO4/Carbon/Reduced Graphene Oxide Nanostructured Composite as a High Capacity and Fast Rate Cathode Material for Rechargeable Lithium Ion Battery

In this study, LiFePO4-carbon (LFP-C) and LFP-C/reduced graphene oxide (rGO) nanocomposites were prepared by ultrasonic spray pyrolysis technique in different calcination conditions to be used as the cathode-active materials for lithium ion battery (LIB). The structure, morphology and composition of the obtained materials were analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM), high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX). The XRD results reveal that the olivine pure phase was obtained after calcination of the LFP-C. The SEM images of the prepared materials exhibit the spherical morphology with nanometer size and also change in the morphology by applying the calcination step. The electrochemical performances of cathode-active materials were investigated by charge–discharge test, electrochemical impedance spectroscopy and cyclic voltammetry. The obtained results for LFP-C show that the electrochemical performance was improved by adding carbon precursor and calcining step; in the optimum calcination conditions; 700 °C for 3 h, the LFP-C shows good results in terms of electrochemical performance in comparison with LFP alone. The LFP-C/rGO nanocomposite exhibits the best electrochemical performance however: highest rechargeable capacity and cycle stability; discharge capacity (168 mAh/g at 0.1 C and 123.5 mAh/g at 10 C) and capacity retention of 100% after 50 cycles with maximum reversibility and lithium ion (Li+) diffusion coefficient. Schematic representation of preparation of the cathode-active materials.

Controlling Li2CuO2 single phase transition to preserve cathode capacity and cyclability in Li-ion batteries

Li2CuO2 is synthesized via a solid-state reaction and its structure and microstructure is characterized using X-Ray Diffraction, Scanning Electron Microscopy and N2 adsorption–desorption isotherms. The capacity of cathode material is evaluated at different preparation conditions to determine the factors affecting charge retention, cyclability, and assuring reproducibility during electrode fabrication. Progressing from previous reports, a maximum capacity retention of 140mAhg−1 is attained in the potential window from 1.5 to 4.2V (Li/Li0) during ten cycles at C/15. The low capacity retention at extended cycling has been associated to the participation of irreversible oxygen redox process evaluated by theoretical calculations and cyclic voltammetry. These processes are minimized when the cycling potential window is confined from 2.0 to 3.8V (Li/Li0), thus, achieving a higher capacity retention up to 100mAhg−1 during 60cycles. Cycling at higher C/rates lowers the capacity down (60mAhg−1 at C/5), but the maximum capacity is restored when returning to C/15. Thus, making Li2CuO2 an attractive material either as active compound or additive in cathodes for Li-ion batteries, as a result of its intrinsic properties such as environmental benign, abundance, cost and straightforward preparation process.

(Invited) Beyond Li-Ion Batteries: Why? , to Where?

The field of advanced batteries faces today two main challenges: development of power sources for electro-mobility (i.e., electrochemical propulsion) and development of storage technologies for load-leveling applications (i.e., large scale batteries). Our starting point in this presentation is state-of-the-art Li-ion batteries. Using carbonaceous anodes and lithiated transition metal cathodes with selected combinations of transition metals may offer very suitable batteries for full EVs. Ni rich Li[NiCoMn]O2, Mn and Li rich Li1+x[NiMnCo]1-xO2 cathode materials operating in the right potential windows can provide Li-ion batteries high enough energy density as needed for full electrochemical propulsion. Also, by combining Li-Ti-O anodes with LiMPO4 olivine cathodes it is possible to demonstrate very suitable battery systems for stationary large energy storage applications. We will review briefly the updated frontier of Li-ion battery technology. Despite the high capability of this battery technology, there are so many applications for power sources in modern life, what promote very strongly a search for more new battery systems. The use of sulfur cathodes in Li batteries can increase considerably their gravimetric energy density, however, the volumetric energy density of Li-sulfur battery systems is not advantageous compared to advanced Li ion batteries. The good news are that in recent years, we see a nice progress in developing durable composite sulfur cathodes. It seems that there are 3 main approaches: encapsulating sulfur in activated carbon matrices, the use of substrates to which the LiSn species formed by sulfur reduction adsorb very strongly and the use of composite cathodes with high loading of sulfur and semi-permeable membranes that avoid transport of LiSn species to the negative electrodes. There is intensive work on Li-oxygen systems. We see progress in development of electro-catalysis for these systems. We will demonstrate electro-catalytic electrodes in Li-oxygen cells, which anodic reactions (Li-peroxide oxidation and molecular oxygen evolution) occur at very low over-potential, that does not endanger the stability of the electrolyte solutions. We demonstrate also red-ox mediators in solutions that facilitate both ORR and OER. Despite the frustration from the slow progress made towards practical directions, it is important to continue investing efforts in Li-oxygen systems. The community should understand that a long-term research is still required in order to see practical horizons to Li-air battery systems. A main problem is the reactivity of all kinds of polar-aprotic solvents towards superoxide and peroxide species formed by oxygen reduction, in the presence of the super-electrophilic Li ions. The intensive work on both Li-S and Li-oxygen systems is connected to a renaissance in efforts to develop practical Li metal anodes for rechargeable batteries. However, intensive studies that were carried out during more than 3 decades, concluded more than 15 years ago that there is no way to cycle Li metal anodes in practical rechargeable Li battery systems. Even if dendrites formation is avoided, continuous detrimental reactions between the active metal and any type of relevant electrolyte solution is inevitable. We will examine briefly what are the chances to overcome the intrinsic problems in using Li metal anodes in practical rechargeable batteries. We may be able to suggest practical alternative anodes for non-aqueous batteries based on sulfur or oxygen cathodes. Discussing other options for “beyond Li-ion batteries” usually includes Na-ion and magnesium batteries. In fact, the plethora of relevant Na-ion insertion cathodes available today is a good surprise, especially since many of them demonstrate excellent kinetics. However, there is no way that Na-ion systems can rival Li-ion battery systems in terms of energy density. Hence, Na-ion systems may be relevant for large energy storage applications, that can benefit from the high abundance of Na in earth crust. However for such uses, demonstrating high stability during prolonged cycling is mandatory. Consequently, a major challenge in R&D of Na-ion batteries is to exhibit the durability required for load-leveling applications. Finally, we will mention the status of R&D of rechargeable Mg battery systems: excellent work was done so far on the anode-solutions part. Several families of electrolyte solutions with wide electrochemical windows, in which Mg anodes are fully reversible were demonstrated. We have not seen yet cathodes materials that function well in the solutions in which Mg anodes are fully reversible, except the low voltage/low capacity Chevrel phase cathode materials. Hence, these systems still require long term research work, especially towards high voltage/high capacity cathode materials. A main incentive to develop rechargeable Mg batteries may relate to advantages in safety and high volumetric energy density.

Enhanced Cycleability of LiNi0.5Mn1.5O4 Cathode with Zn Substitution

Introduction In the last decade, lithium-ion batteries are expected to be applied in electric vehicles. High voltage or capacity cathode materials are required to achieve high energy batteries. Spinel LiNi0.5Mn1.5O4 draws much attention owning to a voltage plateau as high as 4.7 V vs. Li/Li+, accepted capacity, excellent rate capability and good safety.[1] However, high voltage plateau brings side reactions between the cathode and the electrolyte at high voltage which cause capacity fade of spinel LiNi0.5Mn1.5O4.[2] Partial substitution of Mn or Ni by metal elements such as Al, Cr, Fe, Co etc.[3] has been proven to be successful to enhance its cycleability. Series substitutions of Mg instead of Ni are achieved to modify the spinel. The cycleability has been much improved by Mg substitution even charged to 5.1 V. Experimental LiZn x Ni0.5-x Mn1.5O4 was prepared via a sol–gel method assisted by citric acid as chelating agents. Citric acid was firstly dissolved in distilled water. Stoichiometric LiNO3, Zn(NO3)2•6H2O， Ni(NO3)2•6H2O and Mn(NO3)2•4H2O were added to the solution above with continuous stirring. The final solution was evaporated at 80 ºC until a gel was obtained and then dried at 140 ºC for 10 h. The powders was preheated at 400 ºC for 10 h and then calcined at 850 ºC for 24 h in air. The structure and morphology of the samples were studied by X-ray diffraction (XRD). Electrochemical measurements were carried out with CR2032 coin cells. The obtained materials were mixed with C-black and PVDF (80:10:10 wt %) in N-methyl pyrrolidinone solvent. After coating the above slurries on Al foils, the electrodes were dried at 80 ºC in vacuum. In the coin-cell tests, metallic lithium foil was used as the counter and reference electrodes; the electrolyte was 1 M LiPF6in a 1:1 solvent mixture of ethylene carbon (EC)/dimethyl carbonate (DMC). Results and Discussion Figure 1 displays XRD patterns for LiZn x Ni0.5-x Mn1.5O4 (x=0, 0.02, 0.04, 0.08, 0.25, 0.5) powders. The powders all show cubic structure. Rocksalt impurity phase Ni1-x O can be found in the samples of x=0 and 0.02. Impurity Ni1-x O, Li1-x Ni x O, or Ni6MnO8 as a well-known secondary phase in samples of LiZn x Ni0.5-x Mn1.5O4 can be observed at around 37.6˚ and 45.7˚[1–3] when x=0 and 0.02. The impurity intensity decreases with increasing Zn content and the x=0.04 and 0.08 samples show pure spinel phase. While x=0.25 and 0.5, a new impurity phase ZnMn2O4 emerges. In spinel structure, Zn ions prefer occupying tetrahedral sites (8a), such as ZnFe2O4 and ZnAl2O4. As a result, Zn ions are supposed to exist in tetrahedral sites (8a) instead of Li ions. Cycling performance for the electrodes made of LiZn x Ni0.5-x Mn1.5O4 (x=0, 0.02, 0.04, 0.08) at 0.5 C between 3.5–4.9 V is shown in Figure 2. The capacity mainly origin from Ni2+/Ni3+ and Ni3+/Ni4+ redox couples for the spinel LiNi0.5Mn1.5O4. As a result, the capacity decreased with increasing Zn substitution content instead of Ni. However, the cycleability has been much improved by Zn substitution especially when x = 0.08. Only 65 % capacity retention was obtained for the bare electrode. Compared to the pristine electrode, the LiZn0.08Ni0.42Mn1.5O4 electrode delivers 98 mAh/g for the first cycle with remarkable capacity retention of 95 % after 100 cycles. XPS data reveals that a Li-permeable SEI layer formed on the surface of the electrode which protects the electrolyte from oxidation at high voltage[4]. In conclusion, LiZn x Ni0.5-x Mn1.5O4 spinels (x=0, 0.02, 0.04, 0.08, 0.25, 0.5) have been successfully prepared by sol-gel method. Zn substitution significantly improved the cycleability of the spinels. A plausible explanation is that a Li-permeable SEI layer formed on the surface of the Zn substitution electrode which protects the electrolyte from oxidation at high voltage. Figure 1

Oxides As Cathode Materials for Mg-Ion Batteries

Magnesium (Mg) batteries have the potential to revolutionize energy storage. Theoretically these batteries far outperform current Li-ion batteries in both volumetric and gravimetric capacities. [1] Additionally, Mg is a more earth abundant (13.9% Mg in earth’s crust compared to 7x10-4 % Li) and less expensive alternative ($2700/ton Mg and $64000/ton Li), ideal for automobile electrification and thus transforming society. [2] Despite these attractive properties, implementation of Mg-ion batteries has been hindered by several challenges. Most significantly, 1) electrolytes which allow for reversible Mg deposition have limited stability, and 2) reversible intercalation of Mg2+in cathode materials is kinetically slow and often irreversible. Thus, future Mg-ion batteries must incorporate stable high voltage and capacity cathode materials with fast and reversible Mg intercalation kinetics. Mg battery prototypes have used Chevrel phase cathodes, Mo6S8, and its derivatives. [3] These cathodes incorporate a delocalized framework to accommodate the charge and diffusion of hard Mg2+ ions. The multiple Mo ions allow for a diffuse charge network to accommodate intercalated Mg2+ ions. However, Chevrel phase cathodes suffer from low voltages (~1.1 V vs Mg/Mg2+) and only partial reversibility, where the total capacity drops by ~40% after the first discharge cycle. [4] For Mg batteries to be realized commercially, the cathode’s voltage and reversibility must be improved. In this talk we discuss our efforts using transition-metal oxides with open and delocalized structures as cathode materials for Mg-ion batteries. We investigate various bronzes and polyoxometalates, which have known magnesiated forms and high redox potentials. We find that these oxides exhibit reversible electrochemical behavior in various non-aqueous electrolytes and report on our efforts at confining these materials to electrode surfaces. [1] Yoo, H.; Shterenberg, I.; Gofer, Y.; Gershinsky, G.; Pour, N.; Aurbach D. Energy Environ. Sci. 2013, 6, 2265. [2] Saha, P.; Jampani, P.; Datta, M.; Okoli, C.; Manivannan, A.; Kumta, P. J. Electrochem.Soc. 2014, 161, A593. [3] Aurbach, D.; Lu, Z.; Schechter, A.; Gofer, Y.; Gizbar, H.; Turgeman R.; Cohen, Y.; Moshkovich, M.; Levi, E. Nature, 2000, 407, 724. [4] Saha, P.; Datta, M.; Velikokhatnyi, O.; Manivanna, A.; Alman, D.; Kumta P. Prog. Mater. Sci. 2014, 66, 1.

Prospects and Limits of Energy Storage in Batteries.

Energy densities of Li ion batteries, limited by the capacities of cathode materials, must increase by a factor of 2 or more to give all-electric automobiles a 300 mile driving range on a single charge. Battery chemical couples with very low equivalent weights have to be sought to produce such batteries. Advanced Li ion batteries may not be able to meet this challenge in the near term. The state-of-the-art of Li ion batteries is discussed, and the challenges of developing ultrahigh energy density rechargeable batteries are identified. Examples of ultrahigh energy density battery chemical couples include Li/O2, Li/S, Li/metal halide, and Li/metal oxide systems. Future efforts are also expected to involve all-solid-state batteries with performance similar to their liquid electrolyte counterparts, biodegradable batteries to address environmental challenges, and low-cost long cycle-life batteries for large-scale energy storage. Ultimately, energy densities of electrochemical energy storage systems are limited by chemistry constraints.

A Theory of Li Percolation in Rocksalts and Demonstration as Capacity Cathode Materials with Substantial Cation Disorder

There is a clear need to develop dense, lithium intercalation oxides with > 200 mAh/g practical capacity. Today, nearly all high energy density cathodes for rechargeable lithium batteries are well-ordered materials where lithium and other cations occupy distinct sites. Cation-disordered materials are generally disregarded because lithium diffusion tends to be limited in them. The recently demonstrated performance of Li1.211Mo0.467Cr0.3O2, achieving close to 300 mAh/g, shows that lithium diffusion can be facile in disordered materials [1] and made us revisit the question of how Li diffuses through rocksalt-like materials. We have combined ab initio computations of local Li migration barriers with percolation modeling to develop a unified understanding of Li diffusion in close-packed oxides. The theory explains the high capacity of layered and spinel-like materials, and the lack of reversible capacity in γ-LiFeO2. More surprisingly, the new percolation theory also clearly supports that Li-excess is needed to achieve high capacity in partially or fully disordered materials. We can now give very specific guidelines for the amount of Li-excess needed in order to achieve a particular reversible capacity, and open up a new direction for finding very high capacity cathodes.[1] J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries, Science, 343 (6170), 519-522 (2014)