Designing In-Situ-Formed Interphases Enables Highly Reversible Cobalt-Free LiNiO2 Cathode for Li-ion and Li-metal Batteries

•The Co-free LiNiO2 (∼1,050 W h kg−1) retains 80% of capacity after 400 cycles•The robust F- and B-rich CEI in fluorinated electrolyte with LiDFOB is demonstrated•The CEI suppresses side reactions and irreversible structural transformations LiNiO2 (LNO) as the ultimate form of Ni-rich cathode has the lowest price but highest energy density (∼1,050 Wh kg−1), which becomes a good choice for electric vehicle batteries. However, the continuous Ni dissolution, structural disordering, particle cracking, and unstable cathode-electrolyte interphase (CEI) in cycling hinder its practical applications. In this work, we successfully resolved the above challenges by in situ forming a robust fluoride (F)- and boron (B)-rich CEI on LNO using a high-fluorinated electrolyte with LiDFOB additive. The LNO cathode maintains an ultra-high capacity retention of >80% at 400 cycles at high charge cut-off voltage of 4.4 V (versus Li/Li+). The electrolyte also forms an F- and B-rich solid electrolyte interphase on the Li metal/graphite anode, enhancing the Coulombic efficiency of >99% for Li and >99.99 % for graphite. The full cell pairing with graphite anode demonstrated the excellent performance of LNO by exhibiting long-term cycling ability (∼94% after 100 cycles). Cathode materials control both the energy density and cost of Li-ion and Li-metal batteries. The cobalt-free LiNiO2 with relatively low cost and extremely high theoretical energy density (∼1,050 Wh kg−1) is one of the most promising cathode materials for high-energy batteries. However, the continuous Ni dissolution, structural disordering, particle cracking, and unstable cathode electrolyte interphase (CEI) hinder its applications. Here, we surmount these challenges by forming a robust fluoride (F)- and boron (B)-rich CEI on LiNiO2 using a high-fluorinated electrolyte with LiDFOB additive. The LiNiO2 cathode maintains an unprecedentedly high capacity retention of >80% after 400 deep cycles at a high charge cut-off voltage of 4.4 V (versus Li/Li+). In addition, the electrolyte forms an F- and B-rich interphase on the Li metal and graphite anodes, allowing stable cycling of full cells. This work sheds light on designing interfacial chemistry for high-energy cathodes, and its principle is applicable for other alkali metal ion cathodes. Cathode materials control both the energy density and cost of Li-ion and Li-metal batteries. The cobalt-free LiNiO2 with relatively low cost and extremely high theoretical energy density (∼1,050 Wh kg−1) is one of the most promising cathode materials for high-energy batteries. However, the continuous Ni dissolution, structural disordering, particle cracking, and unstable cathode electrolyte interphase (CEI) hinder its applications. Here, we surmount these challenges by forming a robust fluoride (F)- and boron (B)-rich CEI on LiNiO2 using a high-fluorinated electrolyte with LiDFOB additive. The LiNiO2 cathode maintains an unprecedentedly high capacity retention of >80% after 400 deep cycles at a high charge cut-off voltage of 4.4 V (versus Li/Li+). In addition, the electrolyte forms an F- and B-rich interphase on the Li metal and graphite anodes, allowing stable cycling of full cells. This work sheds light on designing interfacial chemistry for high-energy cathodes, and its principle is applicable for other alkali metal ion cathodes. The ever-expanding energy markets from electronic devices and electric vehicles (EVs) to large-scale energy storage systems have stimulated massive investigation of high energy density batteries with long cycle life and low cost.1Bruce P.G. Scrosati B. Tarascon J.M. Nanomaterials for rechargeable lithium batteries.Angew. Chem. Int. Ed. 2008; 47: 2930-2946Crossref PubMed Scopus (5281) Google Scholar, 2Choi J.W. Aurbach D. Promise and reality of post-lithium-ion batteries with high energy densities.Nat. Rev. Mater. 2016; 1Crossref Scopus (2790) Google Scholar Nowadays, the most-used energy storage system is Li-ion batteries (LIBs) consisting of a LiCoO2 cathode (∼140 mAh g−1) and a graphite anode (372 mAh g−1). The capacity and energy density of current Li-ion batteries are always limited by the transition metal cathodes based on intercalation chemistry.2Choi J.W. Aurbach D. Promise and reality of post-lithium-ion batteries with high energy densities.Nat. Rev. Mater. 2016; 1Crossref Scopus (2790) Google Scholar, 3Sun Y.K. Myung S.T. Park B.C. Prakash J. Belharouak I. Amine K. High-energy cathode material for long-life and safe lithium batteries.Nat. Mater. 2009; 8: 320-324Crossref PubMed Scopus (1215) Google Scholar Considering the scarcity and high cost of cobalt metal, Ni-rich lithium nickel manganese cobalt oxides (LiNixMnyCo1−x−yO2, x ≥ 0.5, lithium nickel manganese cobalt oxides [NMCs]) recently have attracted most of the attention for their high theoretical energy density and reasonable cost.4Ceder G. Chiang Y.-M. Sadoway D.R. Aydinol M.K. Jang Y.-I. Huang B. Identification of cathode materials for lithium batteries guided by first-principles calculations.Nature. 1998; 392: 694-696Crossref Scopus (724) Google Scholar, 5Manthiram A. Knight J.C. Myung S.-T. Oh S.-M. Sun Y.-K. Nickel-rich and lithium-rich layered oxide cathodes: progress and perspectives.Adv. Energy Mater. 2016; 6Crossref Scopus (825) Google Scholar, 6Kim J. Lee H. Cha H. Yoon M. Park M. Cho J. Prospect and reality of Ni-Rich cathode for commercialization.Adv. Energy Mater. 2018; 8Google Scholar Among these NMCs, the Co-free layered-type LiNiO2 (LNO) is able to deliver a highest capacity of ∼270 mAh g−1 at a high average voltage (3.8 V versus Li/Li+), which renders EVs with 500-mile driving range in a single charge possible and therefore revives great interests.7Bianchini M. Roca-Ayats M. Hartmann P. Brezesinski T. Janek J. There and back again-the journey of LiNiO2 as cathode active material.Angew. Chem. Int. Ed. 2018; 2018Google Scholar, 8Markevich E. Salitra G. Talyosef Y. Kim U.-H. Ryu H.-H. Sun Y.-K. Aurbach D. High-performance LiNiO2 cathodes with practical loading cycled with Li metal anodes in fluoroethylene carbonate-based electrolyte solution.ACS Appl. Energy Mater. 2018; 1: 2600-2607Crossref Scopus (29) Google Scholar, 9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar, 10Yoon C.S. Choi M.-J. Jun D.-W. Zhang Q. Kaghazchi P. Kim K.-H. Sun Y.-K. Cation ordering of Zr-doped LiNiO2 cathode for lithium-ion batteries.Chem. Mater. 2018; 30: 1808-1814Crossref Scopus (125) Google Scholar, 11Kong F. Liang C. Wang L. Zheng Y. Perananthan S. Longo R.C. Ferraris J.P. Kim M. Cho K. Kinetic stability of bulk LiNiO2 and surface degradation by oxygen evolution in LiNiO2-based cathode materials.Adv. Energy Mater. 2019; 9Crossref Scopus (120) Google Scholar However, the aggressive LNO exhibits much poorer cycling stability and reversibility especially under high cut-off potential (≥4.3 V versus Li/Li+), due to transition metal (Ni) dissolution, particle cracking, surface film thickening, and irreversible phase transformations.7Bianchini M. Roca-Ayats M. Hartmann P. Brezesinski T. Janek J. There and back again-the journey of LiNiO2 as cathode active material.Angew. Chem. Int. Ed. 2018; 2018Google Scholar, 8Markevich E. Salitra G. Talyosef Y. Kim U.-H. Ryu H.-H. Sun Y.-K. Aurbach D. High-performance LiNiO2 cathodes with practical loading cycled with Li metal anodes in fluoroethylene carbonate-based electrolyte solution.ACS Appl. Energy Mater. 2018; 1: 2600-2607Crossref Scopus (29) Google Scholar, 9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar, 12Kalyani P. Kalaiselvi N. Various aspects of LiNiO2 chemistry: a review.Sci. Technol. Adv. Mater. 2016; 6: 689-703Crossref Scopus (241) Google Scholar LNO has a much worse mechanical stability than other NMC cathodes during charge and discharge cycles. At delithiation around 4.15 V (versus Li/Li+), Li1−xNiO2 experiences a large volume change of ∼7% due to phase transition from H2 (rhombohedral phase, 0.55 ≤ x ≤ 0.75) to H3 (0.75 ≤ x ≤ 1.0), inducing significantly high strain and crack formation in LNO particles.9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar, 13Li W. Reimers J.N. Dahn J.R. In situ x-ray diffraction and electrochemical studies of Li1− xNiO2.Solid State Ion. 1993; 67: 123-130Crossref Scopus (627) Google Scholar, 14Dokko K. Nishizawa M. Horikoshi S. Itoh T. Mohamedi M. Uchida I. In situ observation of LiNiO2 single-particle fracture during Li-Ion extraction and insertion.Electrochem. Solidstate Lett. 2000; 3: 125-127Crossref Scopus (89) Google Scholar, 15Kondrakov A.O. Schmidt A. Xu J. Geßwein H. Mönig R. Hartmann P. Sommer H. Brezesinski T. Janek J. Anisotropic lattice strain and mechanical degradation of high- and low-nickel NCM cathode materials for Li-Ion batteries.J. Phys. Chem. C. 2017; 121: 3286-3294Crossref Scopus (352) Google Scholar Such pulverization of particles not only causes loss of electrical contact between primary particles, but also intensifies the parasitic reactions with the electrolyte.16Zhao W. Zheng J. Zou L. Jia H. Liu B. Wang H. Engelhard M.H. Wang C. Xu W. Yang Y. et al.High voltage operation of Ni-Rich NMC cathodes enabled by stable electrode/electrolyte interphases.Adv. Energy Mater. 2018; 8Crossref Scopus (231) Google Scholar, 17Yan P. Zheng J. Liu J. Wang B. Cheng X. Zhang Y. Sun X. Wang C. Zhang J.-G. Tailoring grain boundary structures and chemistry of Ni-rich layered cathodes for enhanced cycle stability of lithium-ion batteries.Nat. Energy. 2018; 3: 600-605Crossref Scopus (468) Google Scholar, 18Mu L. Feng X. Kou R. Zhang Y. Guo H. Tian C. Sun C.-J. Du X.-W. Nordlund D. Xin H.L. et al.Deciphering the cathode-electrolyte interfacial chemistry in sodium layered cathode materials.Adv. Energy Mater. 2018; 8Crossref Scopus (75) Google Scholar Upon further delithiation at a high potential above 4.3 V, Li1−xNiO2 itself became instable. As demonstrated in the LiNiO2-NiO2 phase diagram, the highly delithiated phase (H4 phase) of Li1−xNiO2 with Ni4+ is thermodynamically metastable.19Morales J. Pérez-Vicente C. Tirado J.L. Thermal behaviour of chemically deintercalated Li1− xNi1+ xO2.J. Therm. Anal. 1992; 38: 295-301Crossref Scopus (18) Google Scholar, 20Das H. Urban A. Huang W. Ceder G. First-principles simulation of the (Li–Ni–vacancy) O phase diagram and its relevance for the surface phases in Ni-Rich Li-Ion cathode materials.Chem. Mater. 2017; 29: 7840-7851Crossref Scopus (64) Google Scholar Besides, the Ni3+ tends to migrate into the interslab by replacing Li ions in tetrahedral sites at a high oxidation state. The mixing of Li/Ni ions in the slab and interslab, aggravated by surface dissolution, finally transforms the layered structure of LNO into disordered rock-salts structure (NiO phase).9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar Moreover, since the conventional organic carbonate electrolytes have a limited HOMO (highest occupied molecular orbital), the excess radicals from the oxidation of electrolyte at high voltage (≥4.3 V) tend to corrode the surface of LNO by continual reduction of Ni4+ into Ni2+ with O2 release.9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar, 13Li W. Reimers J.N. Dahn J.R. In situ x-ray diffraction and electrochemical studies of Li1− xNiO2.Solid State Ion. 1993; 67: 123-130Crossref Scopus (627) Google Scholar Extensive efforts have been devoted to ease these issues of LNO. The most effective methods are elemental substitution or doping (Fe,21Prado G. Suard E. Fournes L. Delmas C. Cationic distribution in the Li1− z (Ni1− yFey)1+ zO2 electrode materials.J. Mater. Chem. 2000; 10: 2553-2560Crossref Scopus (27) Google Scholar, 22Mohan P. Kalaignan G.P. Structure and electrochemical performance of LiFex Ni1-xO2 (0.00≤ x≤ 0.20) cathode materials for rechargeable lithium-ion batteries.J. Electroceram. 2013; 31: 210-217Crossref Scopus (13) Google Scholar Ti,23Croguennec L. Suard E. Willmann P. Delmas C. Structural and electrochemical characterization of the LiNi1-yTiyO2 electrode materials obtained by direct solid-state reactions.Chem. Mater. 2002; 14: 2149-2157Crossref Scopus (72) Google Scholar Al,24Kwon S.N. Song M.Y. Park H.R. Electrochemical properties of LiNiO2 substituted by Al or Ti for Ni via the combustion method.Ceram. Int. 2014; 40: 14141-14147Crossref Scopus (19) Google Scholar, 25Liu Z. Zhen H. Kim Y. Liang C. Synthesis of LiNiO2 cathode materials with homogeneous Al doping at the atomic level.J. Power Sources. 2011; 196: 10201-10206Crossref Scopus (45) Google Scholar etc.), surface coating (ZrO2,26Cho J. Kim T.-J. Kim Y.J. Park B. High-performance ZrO2-coated LiNiO2 cathode material.Electrochem. Solidstate Lett. 2001; 4: A159-A161Crossref Scopus (117) Google Scholar Al2O3,27Kang J. Han B. First-principles study on the thermal stability of LiNiO2 materials coated by amorphous Al2O3 with atomic layer thickness.ACS Appl. Mater. Interfaces. 2015; 7: 11599-11603Crossref PubMed Scopus (37) Google Scholar SiO2,28Mohan P. Kalaignan G.P. Electrochemical behaviour of surface modified SiO2-coated LiNiO2 cathode materials for rechargeable lithium-ion batteries.J. Nanosci. Nanotechnol. 2013; 13: 2765-2770Crossref PubMed Scopus (19) Google Scholar etc.), core-shell structure,29Jun D.-W. Yoon C.S. Kim U.-H. Sun Y.-K. High-energy density core–shell structured Li [Ni0. 95Co0. 025Mn0. 025]O2 cathode for lithium-ion batteries.Chem. Mater. 2017; 29: 5048-5052Crossref Scopus (101) Google Scholar gradient strategies,30Sun Y.K. Chen Z. Noh H.J. Lee D.J. Jung H.G. Ren Y. Wang S. Yoon C.S. Myung S.T. Amine K. Nanostructured high-energy cathode materials for advanced lithium batteries.Nat. Mater. 2012; 11: 942-947Crossref PubMed Scopus (829) Google Scholar etc. Recently, Yoon et al. reported that introducing an excess Zr (1.4 atom %) in LNO can form a self-passivating Li2ZrO3 layer on LNO, which improved the capacity retention to 86% after 100 cycles.31Fan X. Chen L. Borodin O. Ji X. Chen J. Hou S. Deng T. Zheng J. Yang C. Liou S.C. et al.Non-flammable electrolyte enables Li-metal batteries with aggressive cathode chemistries.Nat. Nanotechnol. 2018; 13: 715-722Crossref PubMed Scopus (683) Google Scholar The co-doping of Co and Mg in LNO was proved to improve structural and thermal stability by suppressing the anisotropic lattice distortion, thereby enhancing the cycling life to 500 cycles in pouch-type full cells.32Lee M.-H. Kang Y.-J. Myung S.-T. Sun Y.-K. Synthetic optimization of Li [Ni1/3Co1/3Mn1/3]O2 via co-precipitation.Electrochim. Acta. 2004; 50: 939-948Crossref Scopus (544) Google Scholar Unfortunately, under high cut-off voltages (>4.3 V), none of these approaches could improve the long cycling stability of LNO cathode without sacrificing the energy density. Previously, we demonstrated that all-fluorinated electrolyte composing of 1.0 M lithium hexafluorophosphate (LiPF6) in a mixture of fluoroethylene carbonate, 3,3,3-fluoroethylmethyl carbonate, and 1,1,2,2-tetrafluoroethyl-2′,2′,2′-trifluoroethyl ether (FEC: FEMC: HFE, 2:6:2 by weight) can form a fluorinated nanolayer interphase on both Li metal anodes and NMC811 cathodes, which significantly enhanced the Coulombic efficiency (CE) of Li plating and stripping to 99.2% and NMC811 to 99.9%.31Fan X. Chen L. Borodin O. Ji X. Chen J. Hou S. Deng T. Zheng J. Yang C. Liou S.C. et al.Non-flammable electrolyte enables Li-metal batteries with aggressive cathode chemistries.Nat. Nanotechnol. 2018; 13: 715-722Crossref PubMed Scopus (683) Google Scholar Herein, we reported that the fluorinated electrolyte with 2% LiDFOB additive enables more aggressive LiNiO2 cathode to provide a high capacity of >210 mAh g−1 at a charge and discharge rate of 0.5 C and retains more than 80% of the capacity after 400 deep cycles, because the in-situ-formed fluoride (F)- and boron (B)-rich interphase (CEI) on the surface of LNO can withstand a large volume change of high-capacity LNO. In a sharp contrast, only ∼65% capacity of LNO is retained after ∼200 cycles in 1M LiPF6 ethylene carbonate/dimethyl carbonate (EC/DMC) electrolytes. The F- andd B-rich interphase (solid electrolyte interphase [SEI]) on Li metal and graphite anodes also greatly enhanced the cycling stability and CE of Li and graphite anodes. Considering these benefits from the stable CEI and SEI, this work sheds light on rational design of electrolytes to form stable interfacial layer for high-energy batteries. The LiNiO2 microspheres (s-LNO) were synthesized by lithiation of the Ni(OH)2 microspheres with LiOH·H2O at high temperature.9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar, 32Lee M.-H. Kang Y.-J. Myung S.-T. Sun Y.-K. Synthetic optimization of Li [Ni1/3Co1/3Mn1/3]O2 via co-precipitation.Electrochim. Acta. 2004; 50: 939-948Crossref Scopus (544) Google Scholar Figure 1A presents the Rietveld refinement of the powder X-ray diffraction (XRD) pattern of s-LNO, where the unusual background from 15° to 30° is due to the amorphous scattering of Kapton tape that protects the sample from air and moisture. The XRD pattern of s-LNO can be fitted to a 3R-type layered rhombohedral system (space group: R3‾m) with a hexagonal unit cell, and the lattice parameters are a = b = 2.8768(5) Å and c = 14.2025(8) Å, which agrees well with literature.9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar, 12Kalyani P. Kalaiselvi N. Various aspects of LiNiO2 chemistry: a review.Sci. Technol. Adv. Mater. 2016; 6: 689-703Crossref Scopus (241) Google Scholar The inset diagram of Figure 1A illustrates the 3R-type LNO, where edge-sharing NiO6 octahedra forms NiO2 layers stacking in the A-B-C-A-B-C-fashion, and lithium atoms occupy the interstitial octahedral site between NiO2 layers. Scanning electron microscopy (SEM) images in Figure 1B shows the micro-sized (6–10 μm) spherical s-LNO consisting of primary particles with a size of ∼200 nm. The energy-dispersive X-ray spectroscopy (EDS) and high-resolution transmission electron microscopy (HRTEM) results (Figure S1) show the uniform mixing of Ni and O atoms with a ratio of ∼0.5, and clear lattice fringes with a d-spacing of 0.47 nm corresponding to the (003) planes of LNO. The electrochemical performance of LiNiO2 cathode was evaluated under a high charge cut-off voltage of 4.4 V in three different electrolytes: conventional electrolyte of 1.0 M LiPF6 EC/DMC (1:1 by volume, denoted as “E-baseline”); the fluorinated electrolyte of 1.0 M LiPF6 FEC/FEMC/HFE (2:6:2 by weight, denoted as “F-262”); and F-262 with 2 wt % LiDFOB as the additive (“F-262A”). The s-LNO||Li cells in F-262 exhibit an initial discharge capacity of 216 mAh g−1 at 0.5 C (Figure S2A), 1C = 200 mA g−1) as well as high cycling CE of >99.7%. However, the battery with F-262 still shows continuous capacity decay and an increase of cell over potential during cycling. The capacity retention is around ∼88% after 125 cycles (Figure S2B), which is significantly higher than previously reported cycle performance of LNO.8Markevich E. Salitra G. Talyosef Y. Kim U.-H. Ryu H.-H. Sun Y.-K. Aurbach D. High-performance LiNiO2 cathodes with practical loading cycled with Li metal anodes in fluoroethylene carbonate-based electrolyte solution.ACS Appl. Energy Mater. 2018; 1: 2600-2607Crossref Scopus (29) Google Scholar Inspired by the promising results, we further optimized the F-262 electrolyte by adding additional LiDFOB salt to improve the cycling of s-LNO by forming more compact and robust CEI.33Mun J. Lee J. Hwang T. Lee J. Noh H. Choi W. Lithium difluoro(oxalate)borate for robust passivation of LiNi0.5Mn1.5O4 in lithium-ion batteries.J. Electroanal. Chem. 2015; 745: 8-13Crossref Scopus (27) Google Scholar, 34Shui Zhang S.S. An unique lithium salt for the improved electrolyte of Li-ion battery.Electrochem. Commun. 2006; 8: 1423-1428Crossref Scopus (285) Google Scholar, 35Jiao S. Ren X. Cao R. Engelhard M.H. Liu Y. Hu D. Mei D. Zheng J. Zhao W. Li Q. et al.Stable cycling of high-voltage lithium metal batteries in ether electrolytes.Nat. Energy. 2018; 3: 739-746Crossref Scopus (533) Google Scholar, 36Schedlbauer T. Krüger S. Schmitz R. Schmitz R.W. Schreiner C. Gores H.J. Passerini S. Winter M. Lithium difluoro(oxalato)borate: a promising salt for lithium metal based secondary batteries?.Electrochim. Acta. 2013; 92: 102-107Crossref Scopus (70) Google Scholar, 37Schedlbauer T. Rodehorst U.C. Schreiner C. Gores H.J. Winter M. Blends of lithium bis(oxalato)borate and lithium tetrafluoroborate: useful substitutes for lithium difluoro(oxalato)borate in electrolytes for lithium metal based secondary batteries?.Electrochim. Acta. 2013; 107: 26-32Crossref Scopus (36) Google Scholar The amount of LiDFOB additives in the fluorinated electrolyte need to be optimized by balancing the low salt solubility and consumption during cycling. A large amount of LiDFOB cannot be dissolved in the electrolyte and limits the capacity of LNO (Figure S3). As an optimization, 2 wt % of LiDFOB is preferable to add in the F-262A to achieve a high performance of LNO. Meanwhile, the LiDFOB additives can further improve the oxidation stability of fluorinated electrolyte, as demonstrated by extreme low oxidation current up to 6.5 V in Figure S4. The Li plating and stripping curves the Li||Cu cell in F-262A electrolyte (Figure S5A) show a small overpotential of 28 mV and the corresponding CE reaches to 99% after 20 cycles (Figure S5B), which is much better than that in the conventional EC-based electrolytes (<90%). Generally, a CE of 99.9% with 30% excess of Li is required to enable 500-cycle life of Li metal batteries (LMBs), so more work aiming to improve the CE of Li metal to >99.9% is required to enable practical LNO-based LMBs.38Adams B.D. Zheng J. Ren X. Xu W. Zhang J.-G. Accurate determination of coulombic efficiency for lithium metal anodes and lithium metal batteries.Adv. Energy Mater. 2018; 8Crossref Scopus (435) Google Scholar The electrochemical behavior of s-LNO in the F-262A electrolyte was systemically investigated and compared with that in E-baseline electrolyte in half-cell configuration. The quasi-equilibrium potential and overpotential of s-LNO cathode in F-262A electrolyte were investigated in s-LNO||Li coin cells using the galvanostatic intermittent titration technique (GITT). As shown in Figures 1C and S6, s-LNO exhibits a low quasi-equilibrium potential hysteresis of 0.10–0.15 V and ultra-small overpotential of <40 mV during lithiation and delithiation at 1/15 C (13 mA g−1). Since the F-rich CEI has very low electronic conductivity,39Fan X. Ji X. Han F. Yue J. Chen J. Chen L. Deng T. Jiang J. Wang C. Fluorinated solid electrolyte interphase enables highly reversible solid-state Li metal battery.Sci. Adv. 2018; 4: eaau9245Crossref PubMed Scopus (339) Google Scholar the LNO can avoid the continual growth of CEI caused by electrolyte decomposition. The thin F-rich CEI also reduces the interface resistance. The maximum capacity of s-LNO measured using GITT in Figure 1C is 264 mAh g−1, which is comparable with its theoretical capacity of 275 mAh g−1. Moreover, the first three cyclic voltammetry (CV) curves in Figure 1D are overlapped, suggesting the excellent reversibility for the s-LNO electrode in F-262A even though the complex phase transitions take place between rhombohedral and monoclinic phases during cycling. The CV curves at different sweep rates from 0.1 to 5.0 mV s−1 (Figure S7) suggest that redox reaction of s-LNO is limited by Li-ion diffusion process in s-LNO, which is similar to other types of layered LIBs cathodes.40Simon P. Gogotsi Y. Dunn B. Materials science. Where do batteries end and supercapacitors begin?.Science. 2014; 343: 1210-1211Crossref PubMed Scopus (4054) Google Scholar, 41Dong X. Chen L. Liu J. Haller S. Wang Y. Xia Y. Environmentally-friendly aqueous Li (or Na)-ion battery with fast electrode kinetics and super-long life.Sci. Adv. 2016; 2: e1501038Crossref PubMed Scopus (245) Google Scholar, 42Augustyn V. Come J. Lowe M.A. Kim J.W. Taberna P.L. Tolbert S.H. Abruña H.D. Simon P. Dunn B. High-rate electrochemical energy storage through Li+ intercalation pseudocapacitance.Nat. Mater. 2013; 12: 518-522Crossref PubMed Scopus (3355) Google Scholar Figures 1E and 1F present the rate performances of s-LNO in F-262A with different rates from 1/15 C to 5 C (1 C = 200 mA g−1). The corresponding discharging capacities of s-LNO at 1/15, 2/15, 1/3, 1, 2, 3, and 5 C are 265, 250, 224, 192, 168, 148, and 112 mAh g−1 with high average CE of >99.9%, suggesting the excellent rate capability of s-LNO within F-262A. Compared with the E-baseline electrolyte, the s-LNO electrode in F-262A exhibits significantly enhanced long cycling stability with only a minimal increase of cell overpotential after 200 cycles (Figures 1G and S8). The s-LNO in F-262A retains a high specific capacity of 173 mAh g−1 with average CE > 99.9% even after 400 cycles at a current rate of 0.5 C, corresponding to 81% capacity retention (Figure 1H). The CE of LNO electrode with F-262A quickly increases from 85% to >99.9% after several cycles, demonstrating that the CEI formed by oxidation of F-262A can efficiently prevent the electrolyte from further consumption and also protect the LNO from continual Ni dissolution, thereby leading to a high CE in more than 400 cycles. In contrast, the capacity of s-LNO cathode in E-baseline electrolyte drops quickly to less than 150 mAh g−1 over 200 cycles and decays quickly to zero after 215 cycles (Figure 1H). The low initial CE of 65% (versus 85% in F-262A) and fluctuation of CE during cycling imply the severely parasitic reactions and transition metal dissolution of s-LNO in the E-baseline electrolyte.7Bianchini M. Roca-Ayats M. Hartmann P. Brezesinski T. Janek J. There and back again-the journey of LiNiO2 as cathode active material.Angew. Chem. Int. Ed. 2018; 2018Google Scholar, 9Yoon C.S. Jun D.-W. Myung S.-T. Sun Y.-K. Structural stability of LiNiO2 cycled above 4.2 V.ACS Energy Lett. 2017; 2: 1150-1155Crossref Scopus (223) Google Scholar The cycling stability of high-loading s-LNO electrode (∼12 mg cm−2) was also investigated at a low current of 0.1 C. No capacity decay was observed in 20 cycles and high CE of >99.9% was achieved (Figure S9A). However, s-LNO with baseline electrolyte shows a fast capacity decay and low efficiency due to severe side reactions between the electrolyte and s-LNO (Figure S9B). To our best knowledge, the performance of cobalt-free cathode LNO within F-262A electrolyte is the best ever reported for LIBs and LMBs at such high charge cut-off voltage of 4.4 V. The high energy density of LMBs by coupling with s-LNO cathode far exceeds those with any other types of NMC cathodes. To understand the capacity decay mechanism of s-LNO, the evolution of impedances for s-LNO||Li cells with the F-262A and E-baseline electrolytes at different charge and discharge cycles were measured (Figure S10). The semicircle at a high frequency range in the electrochemical impedance spectroscopy (EIS) are attributed to the interfacial resistances (Rint1 + Rint2) of the Li anode and LNO cathode, while the second semicircle in the intermediate frequency could be caused by charge transfer resistance (Rct) on the surface of LiNiO2 cathode.16Zhao W. Zheng J. Zou L. Jia H. Liu B. Wang H. Engelhard M.H. Wang C. Xu W. Yang Y. et al.High voltage operation of Ni-Rich NMC cathodes enabled by stable electrode/electrolyte interphases.Adv. Energy Mater. 2018; 8Crossref Scopus (231) Google Scholar, 31Fan X. Chen L. Borodin O. Ji X. Chen J. Hou S. Deng T. Zheng J. Yang C. Liou S.C. et al.Non-flammable electrolyte enables Li-metal batteries with aggressive cathode chemistries.Nat. Nanotechnol. 2018; 13: 715-722Crossref PubMed Scopus (683) Google Scholar As shown in Figures S10A and S10B, the interfacial resistance of (Rint1 + Rint2) in these two electrolytes decreases from the 0th to 20th cycles due to the breaking of highly resistive passivation layer (Li2CO3) on Li and reforming of ionic conductive CEI and SEI films on electrodes. After 20 cycles, the interfacial resistance (Rint1 + Rint2) and charge transfer resistance (Rct) of LNO in F-262A electrolytes is almost constant thanks to the protection of stable SEI and CEI films. However, the rapidly increased interfacial resistance (Rint1 + Rint2) and charge transfer resistance (Rct) of LNO in the E-baseline electrolyte manifests a continuously growing SEI and CEI films. The Li||LiNiO2 cells after 100 cycles in F-262A and baseline electrolytes were disassembled. The surfaces of Li metal anode and LNO cathode in F-262A are clean and intact (Figure S10C). On the contrary, the Li metal anode after 100 cycles in baseline electrolyte was covered by a thick layer and a lot of black compounds were stuck on the separator (Figure S10D). The inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis demonstrated that the black compounds contain high content of Li and Ni. Therefore, serious side reactions and continual Ni dissolution of LNO occurred in E-baseline electrolyte (Figure S10E). Meanwhile, a much cleaner separator was observed, and only a very small amount of Ni was identified by ICP-OES in F-262A electrolyte. Th