Ultrahigh energy density Li-ion batteries based on cathodes of 1D metals with –Li–N–B–N– repeating units in α-LixBN2 (1 ⩽ x ⩽ 3)

Abstract

Ultrahigh energy density batteries based on α-Li x BN 2 (1≤x≤3) positive electrode materials are predicted using density functional theory calculations. The utilization of the reversible LiBN 2 + 2 Li + + 2 e − Li 3 BN 2 electrochemical cell reaction leads to a voltage of 3.62 V (vs Li/Li + ), theoretical energy densities of 3251 Wh/kg and 5927 Wh/L, with capacities of 899 mAh/g and 1638 mAh/cm 3 , while the cell volume of α-Li 3 BN 2 changes only 2.8 % per two-electron transfer. These values are far superior to the best existing or theoretically designed intercalation or conversion-based positive electrode materials. For comparison, the theoretical energy density of a Li-O 2 /peroxide battery is 3450 Wh/kg (including the weight of O 2 ), that of a Li-S battery is 2600 Wh/kg, that of Li 3 Cr(BO 3 )(PO 4 ) (one of the best designer intercalation materials) is 1700 Wh/kg, while already commercialized LiCoO 2 allows for 568 Wh/kg. α-Li 3 BN 2 is also known as a good Li-ion conductor with experimentally observed 3 mS/cm ionic conductivity and 78 kJ/mol (≈ 0.8 eV) activation energy of conduction. The attractive features of α-Li x BN 2 (1≤x≤3) are based on a crystal lattice of 1D conjugated polymers with -Li-N-B-N-repeating units. When some of the Li is deintercalated from α-Li 3 BN 2 the crystal becomes a metallic electron conductor, based on the underlying 1D conjugated π electron system. Thus α-Li x BN 2 (1≤x≤3) represents a new type of 1D conjugated polymers with great potential for energy storage and other applications. IntroductionThere is a quest for materials that would allow for storing large amounts of energy per unit weight and volume and can be utilized as electroactive species in electrochemical energy storage devices. These materials typically serve as part of the positive electrode of batteries while negative electrodes may be composed of bulk metals, such as Li, Na, Mg, Al, etc, or as electrically and ionically conductive composite materials containing these metals. The driving force of the discharge process in batteries is the chemical potential difference of electrons and mobile cations in the positive and negative electrodes manifesting as voltage between the current collectors. Main stream battery research focuses on improving Li-ion batteries that are based on positive electrode materials capable of intercalating/deintercalating Li + ions during the charge/discharge processes. . In practice, these values are far smaller due to additional materials that are needed for practical implementations of the corresponding electrochemistries. For example, best realized Li-S batteries allow for 300-500 Wh/kg 3 which is still significantly better than that of commercially available batteries (130-200 Wh/kg, based on LiMO 2 ). For intercalationbased cathode materials there is typically a factor of 3-4 difference between the theoretical and practical energy density values. Research is also conducted on metal-air type batteries that have extremely large theoretical energy densities. A rechargeable Li-O 2 /peroxi...

Li3BN2 as a Transition Metal Free, High Capacity Cathode for Li‐ion Batteries

“…The results from the pure CB half‐cells indicate that CB only contributes to less than 0.5 % of the total storage capacity (see Supporting Information for details). Note that the first discharge capacity of 890 mA h g −1 is very close to the theoretical capacity (899 mA h g −1 ) based on the following electrochemical reaction predicted from the DFT calculation …”

“…In this study, we have investigated a completely new class of cathode materials that eliminate the need of using transition metals as they are built from light elements only, such as B, N and C. Recent studies through density functional theory (DFT) calculations have predicted a new class of cathode materials based on functionalized hexagonal boron nitride (FBN) materials . The DFT calculations reveal that functionalization of boron nitrides can tune the electrochemical potentials of FBN materials and create intercalation‐type materials with ultrahigh specific capacities and unprecedented energy densities because of the elimination of heavy transition metals . The charge storage mechanism of FBN materials is predicted to be accomplished by redox reactions due to the valence state change of N ions .…”

“…The DFT calculations reveal that functionalization of boron nitrides can tune the electrochemical potentials of FBN materials and create intercalation‐type materials with ultrahigh specific capacities and unprecedented energy densities because of the elimination of heavy transition metals . The charge storage mechanism of FBN materials is predicted to be accomplished by redox reactions due to the valence state change of N ions . Inspired by these predictions, we have synthesized Li 3 BN 2 powder via reaction between Li 3 N and h‐BN at 900 °C (see Supporting Information for experimental details) and carried out the first‐ever electrochemical investigation of Li 3 BN 2 (a 3D member of FBN materials that can be derived from Li 3 N functionalized 2D h‐BN) for its potential as a transition metal free, high‐capacity cathode material for Li‐ion batteries.…”

“…Li/Li + and the second anodic peak at ∼2.23 V. These two anodic peak positions are very close to the two voltage plateaus during charge, unambiguously indicating that charging of α‐Li 3 BN 2 proceeds in two oxidation steps. Based on the DFT calculations, we hypothesize that the first oxidation corresponds to the valence state change of N ions in the BN 2 anion from 3 − to 2.5 − , whereas the second oxidation is due to the further change of N ions from 2.5 − to 2 − . This hypothesis is consistent with our X ‐ ray photoelectron spectroscopy (XPS) and soft X‐ray absorption spectroscopy (sXAS) analyses to be shown later.…”

“…As shown in Figure (a), the N1s spectra of these samples are quite broad and unsymmetric, suggesting overlap of several peaks. Based on the charge/discharge profile (Figure ) and the resonance structures of the BN 2 3− anion and its gradually delithiated versions of Li 2 BN 2 and LiBN 2 predicted via DFT calculations, 3 major peaks in each N1s spectrum can be fitted and attributed to BN 2 3− , BN 2 2− and BN 2 1− resonant groups. It is known that the N1s spectrum of a substance with covalent bond and/or negative valence state is normally peaked at <400.0 eV, such as N in h‐BN and Li 3 N .…”