Industrial challenges on the development of Si-based anodes remain vivid since no Li-ion battery using significant amount of Si has been commercialized so far due to the issues to be overcome, mainly related to the high volume change and to cycling abilities. The project NEWMASTE aims to face this challenge through the development of new nanocomposite Si/Intermetallic materials with stabilized surface against electrolyte decomposition
NEWMASTE is a 3-year multidisciplinary project involving three academic and two industrial companies. The main aim is to develop a newly discovered Si-based nanostructured composite and to scale-up the optimized material for being used as negative electrode in Li-ion batteries. This material has been patented in Europe (EP2242129A1) and US (US 20100270497). The nanostructured composite at least triples the capacity of current graphite electrodes. The main steps of this development are: <br /> <br />1.- the optimization of composition and morphology of the composite and the improvement of its interface with the electrolyte to obtain a high energy density battery with long cycle life. The improvement of faradic efficiency is a key issue. <br />2.- the technical processes for composite material synthesis and its surface treatment. <br />3.- the electrochemical cell design which includes: <br /> i) understanding of the capacity fading during cycling and safety assessment <br /> ii) definition of the electrode manufacturing process parameters <br /> iii) assessment of the material performance, and life in real electrochemical cell <br /> iv) evaluation of coupling to intermittent energy sources for electric storage <br />4.- the technico-economic assessment of the different processes <br />5.- the assessment of the recycling feasibility and the Life Cycle Analysis of the new material and the battery. <br />
As a starting step of the project, a composite material named as «reference« consisting of Si nanoparticles (~ 200 nm) surrounded by a matrix of Ni3.4Sn4, C and Al was synthesized by mechanical milling. This nanocomposite was prepared in two steps: first the synthesis of intermetallic Ni3.4Sn4 by powder metallurgy followed by mechanical milling of all components. Structural and chemical properties of this material were analyzed by XRD, SEM, TEM, XPS and Mössbauer spectroscopy.
Although the electrochemical properties of this material are highly interesting (Crev > 600 mAh/g and 200 cycles with a coulombic efficiency of 99.2 %), its performances must be further improved for its industrial use. Thus, in order to improve its mechanical stability upon cycling by enhanced nanostructuration, the intermetallic compound Ni3.4Sn4 can be replaced by a two-phase mixture Ni3.4Sn4 + Ni3Sn2. This approach also reduces the content of Sn, an expensive element. Also, we investigated the replacement of the nanometric Si-precursor by micrometric Si trade to reduce manufacturing costs.
Improving the coulombic efficiency of this material was followed by surface treatment processes and by adapting the composition of the electrolyte. Several surface treatments (hydroxylation and phosphate coating) on the silicon nanoparticles and the composite material were carried out. As for the electrolyte formulation, studies are underway to replace conventional solvents (DMC , PC) by other organic solvents (THF ) as well as by ionic liquids . Similarly, the addition of additives (FEC and VC) to enhance the passivation layer that is formed on the electrode in the first cycle is evaluated.
Finally, we seek to adapt the methods of synthesis and surface treatment for the fabrication of materials at pilot scale and to optimize the electrode and the complete cell technology for real applications.
A reference composite material consisting of Si nanoparticles (~ 200 nm), surrounded by a matrix of Ni3.4Sn4 , C and Al was synthesized in a reproducible way by mechanical milling. A method for morphological analysis by SEM microscopy cross-section was developed to analyze the homogeneity of the matrix, a necessary condition to ensure good mechanical stability of the material during electrochemical cycling.
The electrochemical properties of this material were determined by galvanostatic cycling. In the first cycle, it provides a specific capacity of 1000 mAh/g with irreversible capacity of 20%. The cycling capacity is ~ 650 mAh/g with a loss of ~ 23% for 200 cycles and a coulombic efficiency of 99.2%. The nanometric Si precursor was replaced by micrometric Si trade to reduce manufacturing costs. Interestingly, the specific capacity is not diminished and the coulombic efficiency increases to 99.4%. Moreover, the use of electrolyte-enriched FEC allows further increasing of the reversible capacity about 20% and coulombic efficiency up to 99.5%.
Surface treatments were performed on the precursor Si (hydroxylation) and on the reference composite (phosphate coating). Hydroxylation improves silicon cycle-life while phosphating increases the coulombic efficiency of the composite up to 99.7%. As for the development of new electrolytes, it has been shown that a mixture THF-LiTFSI (without additive) gave very interesting results. Capacities greater than 600 mAh/g are obtained at 2C regime.
Studies on the electrode technology for real applications were conducted. In particular, the passage from a laboratory-type electrode formulation (55: 25: 20% composite: CMC: carbon) to an industrial one was evaluated (87: 8: 5). This change induces a very modest capacity loss (~ 10%).
Halfway through the NEWMASTE project, the optimized capacity of the reference material (Cinitial = 1050 mAh/g, Cirrev = 240 mAh / g) is close to the project objectives (Cinitial = 1000 mAh/g, Cirrev = 200 mAh/g) when FEC-rich electrolytes and a laboratory-type electrode formulation are used. The cycling stability is also close to the targets: 20% for 200 cycles. To better understand the aging phenomena and further improve these properties morphological analysis will be carried out at different states of electrode cycling.
As regards the coulombic efficiency, the best results obtained (e = 99.7% after passivation treatment) are still below target (e = 99.95%). This is a key parameter because minimum lithium consumption during electrochemical cycling should be guaranteed to use this material in complete cell. To overcome this challenge two avenues of investigation are considered: i) the search for an optimal combination between the nature of the passivation layer and the formulation of the electrolyte and ii) the total or partial replacement of the intermetallic Ni3.4Sn4 precursor by other compounds resulting in a less significant decomposition of the electrolyte.
The most promising materials of the project will be tested in full battery configuration and cylindrical laboratory accumulators to approximate the conditions of industrial storage tests. The optimal protocol for the manufacture of the electrode and a technical-economic evaluation processes will be established. The coupling of electrochemical cells with intermittent energy sources for electric storage will be evaluated.
Three publications and a patent in preparation
NEWMASTE is a 3-year multidisciplinary project involving 3 academic and 2 industrial companies. NEWMASTE aims to develop a newly discovered Si-based nanostructured composite and to scale-up the optimized material for being used as negative electrode in Li-ion batteries*. The nanostructured composite at least triples the capacity of current graphite electrodes. The main steps of this development are:
1.- the optimization of composition and morphology of the composite and the improvement of its interface with the electrolyte to obtain a high energy density battery with long cycle life. The improvement of faradic efficiency is a key issue.
2.- the technical processes for composite material synthesis and its surface treatment.
3.- the electrochemical cell design which includes:
i) understanding of the capacity fading during cycling and safety assessment
ii) definition of the electrode manufacturing process parameters
iii) assessment of the material performance, and life in real electrochemical cell
iv) evaluation of coupling to intermittent energy sources for electric storage
4.- the technico-economic assessment of the different processes
5.- the assessment of the recycling feasibility and the Life Cycle Analysis of the new material and the battery.
This ambitious project is a real breakthrough in terms of electrochemical performance compared to the today Li-ion batteries which has motivated a Belgian industrial partner (Umicore) to work on the scale-up of this material on their own funding. Moreover, it is based on an original approach to develop and produce new high capacity and long life Li-ion battery anode materials. In 2012, a closely related project named NAMASTE was submitted to PROGELEC Call. It was ranked in complementary list (i.e. to be financed if funds available) and ranked just after the last funded proposal on electrochemical storage. It received a highly positive evaluation on consortium quality, methodological approach, industrial support, scientific and economic impact. The partners of NEWMASTE do believe in the feasibility and success of this research project and have pursued in 2012 their efforts to get convincing preliminary results. These results have been published in two recent scientific publications and new very promising results have been obtained on new composites which increases considerably the chance of success of the novel project: NEWMASTE. Last but not least, industrial challenges on the development of Si-based anodes remain vivid since, in spite of numerous announcements, no Li-ion battery using significant amount of Si has been commercialized so far due to the issues to be overcome, mainly related to the high volume change and to cycling abilities. The project NEWMASTE aims to face this challenge.
*This material has been patented in US and EU and co-filed by French Research Institution (CNRS) and industry (SAFT). EP2242129 (A1) (20/10/2010) US 20100270497 (28/10/2010).
Monsieur Fermin CUEVAS (Institut de Chimie et des Matériaux Paris Est, Equipe CMTR)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ICMPE/CMTR Institut de Chimie et des Matériaux Paris Est, Equipe CMTR
SAFT SAFT SAS
PCM2E Université F. Rabelais,Tours, PCM2E
ICGM/AIME Institut Charles Gerhardt Montpellier, Equipe AIME
Help of the ANR 707,428 euros
Beginning and duration of the scientific project: January 2014 - 42 Months