Laure Monconduit

Beyond Intercalation‐Based Li‐Ion Batteries: The State of the Art and Challenges of Electrode Materials Reacting Through Conversion Reactions

Despite the imminent commercial introduction of Li-ion batteries in electric drive vehicles and their proposed use as enablers of smart grids based on renewable energy technologies, an intensive quest for new electrode materials that bring about improvements in energy density, cycle life, cost, and safety is still underway. This Progress Report highlights the recent developments and the future prospects of the use of phases that react through conversion reactions as both positive and negative electrode materials in Li-ion batteries. By moving beyond classical intercalation reactions, a variety of low cost compounds with gravimetric specific capacities that are two-to-five times larger than those attained with currently used materials, such as graphite and LiCoO(2), can be achieved. Nonetheless, several factors currently handicap the applicability of electrode materials entailing conversion reactions. These factors, together with the scientific breakthroughs that are necessary to fully assess the practicality of this concept, are reviewed in this report.

Better Cycling Performances of Bulk Sb in Na-Ion Batteries Compared to Li-Ion Systems: An Unexpected Electrochemical Mechanism

Pure micrometric antimony can be successfully used as negative electrode material in Na-ion batteries, sustaining a capacity close to 600 mAh g(-1) at a high rate with a Coulombic efficiency of 99 over 160 cycles, an extremely high capacity compared to any other compound tested against both Li and Na. The reaction mechanism with Na does not simply go through the alloying mechanism observed for Li where the intermediate species are those expected from the phase diagram. In the case of Na, the intermediate phases are mostly amorphous and could not be precisely identified. Surprisingly, we evidenced that a competition takes place at the end of the discharge of the Sb/Na cell between the formation of the hexagonal and the cubic polymorphs of Na(3)Sb, the last being described in the literature as unstable at atmospheric pressure and only synthesized under high pressure (1-9 GPa). In addition, fluoroethylene carbonate added to the electrolyte combined with an appropriate electrode formulation based on carboxymethyl cellulose, carbon black, and vapor ground carbon fibers seems to be determinant in the excellent performances of this material.

NiP3: a promising negative electrode for Li- and Na-ion batteries

Due to the abundance and low cost of sodium-containing precursors ambient temperature sodium ion batteries are promising for large scale grid storage. The low melting point of Na (97.7 °C) compared to 180.6 °C for Li represents a significant safety hazard for the use of Na metal anodes at ambient temperatures, which emphasizes the need for scientists and engineers to identify, design and develop new negative electrodes for Na-ion batteries. The identification of a suitable negative electrode is a crucial challenge for any further successful development of new cells, and to date efficient and competitive negative electrodes for NaB are still very rare. In this work we demonstrate that NiP3 could be a good challenger for this purpose. NiP3 based electrodes are evaluated as negative electrode materials for Li-ion batteries (LiB) and Na-ion batteries (NaB). The study of the reaction mechanism reveals the formation of a phase of composition close to Li3P and Na3P embedding Ni nanoparticles as the final reaction product after a full discharge. While the direct conversion of NiP3 into Na3P is identified for the reaction versus Na, it is still unclear whether an amorphous phase exists during the first discharge for the reaction versus Li before the conversion. Furthermore, thanks to the carboxymethyl cellulose/carbon black (CMC/CB) electrode formulation, the NiP3 electrode possesses a very promising capacity with a reversible storage capacity higher than 1000 mA h g−1 after 50 cycles for LiB and 900 mA h g−1 after 15 cycles for NaB, which represents one of the highest capacities ever sustained in Na-ion batteries.

Electrochemical Reactivity of Cu3 P with Lithium

Copper phosphide, Cu 3 P, has been synthesized using a solvothermal route. The electrochemical reaction of this binary phosphide with lithium has been studied to describe the insertion/extraction mechanism. The discharge of the half-cell Cu 3 P/Li up to 0.1 V proceeds by lithium insertion into a Cu 3 P hexagonal structure and then by two successive biphasing processes involving the formation of LiCu 2 P and Li 2 CuP, respectively. Simultaneously, copper metal is extruded from the matrix. During the charge the reversible processes are observed. After 15 cycles a 400 mAh g -1 specific capacity is retained.

Nanoconfined phosphorus in mesoporous carbon as an electrode for Li-ion batteries: performance and mechanism

Phosphorus–mesoporous carbon composites have been prepared by direct vaporization–condensation of phosphorus onto mesoporous carbon. The as-prepared P1–C1 composite showed improved performance in batteries vs. Li compared with classical P mainly due to the electronic modification of the nanoconfined P. By combining the electrochemical analysis with 31P and 7Li NMR, in situ X-ray diffraction and Raman spectroscopy we have characterized the electrochemical mechanism of the P1–C1 composite vs. Li in the battery. Such activated P is able to react reversibly with Li to form Li3P and this reaction takes place at higher potential than that of classical P. This study demonstrates that the effects of nano-confinement of an active material (AM) (here P) on mesoporous carbons paves a way (i) to stabilize different polymorphs of an active element (vs. Li) that present a modified reactivity vs. Li and (ii) to enhance the electrochemical performance of AM vs. Li. Last but not least it is demonstrated that the nature of carbon is determinant for the electrochemical performance.

Tracking Sodium-Antimonide Phase Transformations in Sodium-Ion Anodes: Insights from Operando Pair Distribution Function Analysis and Solid-State NMR Spectroscopy

Operando pair distribution function (PDF) analysis and ex situ 23Na magic-angle spinning solid-state nuclear magnetic resonance (MAS ssNMR) spectroscopy are used to gain insight into the alloying mechanism of high-capacity antimony anodes for sodium-ion batteries. Subtraction of the PDF of crystalline NaxSb phases from the total PDF, an approach constrained by chemical phase information gained from 23Na ssNMR in reference to relevant model compounds, identifies two previously uncharacterized intermediate species formed electrochemically; a-Na3–xSb (x ≈ 0.4–0.5), a structure locally similar to crystalline Na3Sb (c-Na3Sb) but with significant numbers of sodium vacancies and a limited correlation length, and a-Na1.7Sb, a highly amorphous structure featuring some Sb–Sb bonding. The first sodiation breaks down the crystalline antimony to form first a-Na3–xSb and, finally, crystalline Na3Sb. Desodiation results in the formation of an electrode formed of a composite of crystalline and amorphous antimony networks. We link the different reactivity of these networks to a series of sequential sodiation reactions manifesting as a cascade of processes observed in the electrochemical profile of subsequent cycles. The amorphous network reacts at higher voltages reforming a-Na1.7Sb, then a-Na3–xSb, whereas lower potentials are required for the sodiation of crystalline antimony, which reacts to form a-Na3–xSb without the formation of a-Na1.7Sb. a-Na3–xSb is converted to crystalline Na3Sb at the end of the second discharge. We find no evidence of formation of NaSb. Variable temperature 23Na NMR experiments reveal significant sodium mobility within c-Na3Sb; this is a possible contributing factor to the excellent rate performance of Sb anodes.

Exceptionally highly performing Na-ion battery anode using crystalline SnO2 nanoparticles confined in mesoporous carbon

Confined and unconfined SnO2 nanoparticles in the pores of mesoporous carbon were prepared and tested as anode materials vs. Na. Both composites present small crystalline SnO2 particles (∼3 nm) but different location and dispersion in the carbon matrix. When the particles are homogeneously distributed and confined in the carbon pores, an initial reversible capacity of 780 mA h g−1 is achieved with unprecedented capacity retention of 80 and 54% after 100 and 4000 cycles, respectively, at a high current rate (50 C, 1800 mA g−1). Unexpectedly, over two current rate variation cycles from 1 C to 500 C, the composite recovers 81% and 97%, respectively after returning from the 500 C to the 1 C rate. To our knowledge, no other material with such a long cycling life and superior performance in terms of capacity and rate capability has been reported so far for sodium ion batteries. HRTEM, XRD, N2 adsorption, XPS and galvanostatic cycling results suggest that confined SnO2 particles undergo an enhanced sodium alloying/dealloying process due to their special confinement inside the pores, which increases their conductivity, facilitates the diffusion of Na+ ions and buffers the large volumetric changes during charge/discharge. These high performances cannot be delivered when SnO2 is not confined and not well dispersed in the carbon pores. This work demonstrates that nano-confinement of anode species in carbon is a valuable concept affording the modification of the fundamental properties of guest species along with their electrochemical performances leading to highly stable and performing materials with a long life for Na-ion batteries.

The good reactivity of lithium with nanostructured copper phosphide

In Li-ion battery technology, Li diffusion in the electrode is mainly limited by the quality of the interfaces. To take advantage of the large capacity gain offered by the transition metal phosphides (TMP) as negative electrode, a new self-supported TMP/Cu nanoarchitectured electrode concept is proposed. This specific design allows one to fine-tune control of both (TMP)/current collector and (TMP)/electrolyte interfaces of the electrode. This new electrode preparation process is based on an electrochemical templated synthesis of copper nanorods followed by a phosphorus vaporization. The P vapour reacts with the Cu nanorods leading to Cu3P nanorods. Preliminary electrochemical tests of the as-obtained Cu3P nanorods/Li half cell show the great interest of using such a nanostructured TMP electrode in a Li battery. These nanoarchitectured phosphide electrodes can sustain a C-rate (a full discharge in 1h) cycling without exhibiting any important reversible capacity loss for 20 cycles.

TiSnSb a new efficient negative electrode for Li-ion batteries: mechanism investigations by operando-XRD and Mössbauer techniques

We report the electrochemical study of TiSnSb towards Li, as a negative electrode for Li-ion batteries. TiSnSb can reversibly take up more than 5 lithiums per formula unit leading to reversible capacities of 540 mA h g−1 and 4070 mA h cm−3 at 2 C rate. From complementary operandoXRD and Mossbauer spectroscopy measurements, it was shown that during the first discharge the TiSnSb undergoes a conversion process leading simultaneously to the formation of Li–Sb and Li–Sn alloys. At the end of the discharge, Li3Sb and Li7Sn2 were identified. Once the first discharge is achieved, both phases were shown to form Ti–Sn or Ti–Sb or Ti–Sn–Sb nanocomposites. The cycling performance of TiSnSb was shown to be excellent with maintaining 90% of the specific capacity during 60 cycles at 2 C rate. The good electrochemical performance of TiSnSb (compared to Sn and Sb) seems to be a consequence of the presence of the non-active metal. The comparative study of Ti/Sn/Sb composite demonstrated that the structural feature of the pristine material clearly impacts both the mechanism involved during the cycling and the corresponding performance.

Improvement of intermetallics electrochemical behavior by playing with the composite electrode formulation

The impact of both various binders and carbon additives on the electrochemical behavior of intermetallics-based (FeSn2, NiSb2, TiSnSb) negative electrodesvs.Li was evaluated and accurately studied for FeSn2. The formulation of the composite electrode allowed enhancement of both the capacity retention as well as the rate capability. CMC/VGCF (carboxymethyl cellulose/vapor grown carbon fiber) used as a binder/conductive additive allows retention of 100% of the specific capacity during 35 cycles at 2C rate (2Li h−1), whereas the fading is dramatic after only a few cycles at a low rate with classical powdered electrodes. Notably, an extra capacity is observed with CMC/VGCF. In the case of FeSn2, it is shown that the improvement of performance achieved with CMC/VGCF results from better efficiency of the conductive additive to form an electronic percolation web around the active material (AM) particles rather than from the buffering of the volume variations of the AM particles, contrary to the case of Si-based electrodes. The improved cycle life is also likely due to the better ability of CMC to cover the particles compared to PVDF (polyvinylidene fluoride), resulting not only in stronger interparticle bonding but also in a better SEI layer. It is suggested that the growing of an insulating SEI layer by the degradation of the liquid electrolyte is an important factor in the fading mechanism of FeSn2 composite electrodes. Finally, the aqueous processing of the FeSn2, NiSb2, and TiSnSb intermetallics-based composite electrodes is feasible.