Patent Application
20180175371
The present invention is concerned with providing a nanowire structure for use as an electrode in a lithium ion battery. More particularly, the invention is concerned with providing a nanowire structure for use as an electrode which can be tailored to suit the application in which the battery is intended to be used.
Li-ion batteries are used in a wide spectrum of applications that have very different power requirements. For instance, some applications need power to be delivered quickly, such as for example power tools that often require very high discharge rates. Other applications, such as mobile phones, are discharged at relatively slower rates. In such cases having a high capacity is the priority as it prolongs the time between charges.
Due to the ever increasing demand for portable electronic devices and the rapidly expanding electric vehicle market, it has become very desirable to produce a high-capacity electrode material that can increase the energy density of lithium-ion batteries. Li-alloying materials such as Silicon (Si) and Germanium (Ge) have emerged as the forerunners to replace the current, relatively low-capacity, carbonaceous based anodes. Germanium and silicon undergo a volume expansion and contraction during the charge and discharge processes, which creates large mechanical stresses within the electrode. When bulk films and micrometre-sized particles are used as electrodes in lithium-ion batteries, capacity fading occurs, due to pulverization and the loss of electrical contact between the active material and the current collector. This results in a short battery life time for bulk electrodes.
The benefit of using pure Si electrodes is that it has the highest known specific capacity of any material and also is inexpensive. However, all pure Si electrodes suffer from the major disadvantage of a relatively poor rate capability due to the inherently low conductivity of Si and slow rates of Li+ diffusivity. In an attempt to overcome this problem, nanowire (NW) based arrangements have been utilised with some success. When grown directly from the current collector, the NW morphology outperforms bulk materials, as each individual NW remains electrically contacted, despite the large volume fluctuations occurring as a result of constant alloying and dealloying. The ease of processability and uncomplicated, binder-free nature of such electrodes make them highly attractive as next-generation Li-ion anode materials. In an attempt to extend the cycle life, more complex binder-free NW arrangements have also been fabricated. These include carbon-silicon and crystalline-amorphous core-shell structures, NWs coated with conductive inactive materials and NWs grown via AAO template assisted growth. Although all of these systems show some improvements in capacity retention, they suffer from the same major disadvantage of all Si electrodes, namely that of a relatively poor rate capability due to the inherently low conductivity of Si and slow rates of Li+ diffusivity.
A further problem encountered with Si NW electrodes is the continuous growth of the passivating SEI layer. The formation of a stable, thin SEI layer in the initial cycles is essential, as it prevents continuous decomposition of the electrolyte during each charge process. However, if the SEI is not flexible and cohesive, then the continuous expansion and contraction of the NWs during cycling can lead to cracking of the passivating layer. This re-exposes the active material causing further decomposition of the electrolyte on the surface, and thickening of the SEI layer with each cycle. This ultimately inhibits the performance of the NWs over extended cycles as it increases the cell's internal impedance on the surface of the active material due to the decomposition of the electrolyte.
Ge NW electrodes show a much superior rate capability over Si, due to the high conductivity and high rate of diffusivity of Li, thus making them suitable for high-rate applications. Furthermore Ge NW electrodes exhibit high capacities and very long cycle lives in long term cycling experiments at rates of 1C. The low capacity fade of these electrodes is due to the transformation of the NWs over the initial cycles into a sponge like network of Ge ligaments which remains well contacted to the current collector despite the volume changes occurring. However, the drawback of Ge NW electrodes is that they have a lower capacity and are more expensive to fabricate than Si electrodes.
Traditional methods for preparing electrodes are multi-step processes. The active material is first mixed with conducting carbon and a nonconducting polymeric binder (such as polyvinylidene fluoride) in an inorganic solvent (NMP). The resulting slurry is coated onto a current collector, pressed and then annealed for several hours. Such preparation methods are not ideal, as the whole process is time consuming and the solvent used in the slurry formulation is expensive, toxic and generates flammable vapors. Another major disadvantage is that the binder and conductive additives are electrochemically inactive, yet still add mass to the electrode. This results in a limit of the achievable capacity.
The paper entitled “Growth of Ge—Si nanowire heterostructures via chemical vapour deposition”, published in the Thin Solid Films publication, vol. 519 no. 13, 12 Feb. 2011, discloses the growing of Ge NW stems from a Si wafer via a gold seeded chemical vapour deposition mechanism, with the migrated gold acting as a seed for Si NW branch growth. However, there are a number of disadvantages of the resulting heterostructure were it to be used as an electrode in a lithium ion battery. The heterostructure formed by this technique results in Si branches only growing from the top of the Ge NW stems. This means that there would be very little interconnection between neighbouring NWs and branches, which is necessary for the stable cycling of Li-alloying materials such as Ge and Si in a Li-ion battery, where the NWs must undergo a complete restructuring to form a continuous porous network that is mechanically robust. Secondly, the described technique does not provide any control over the diameter of the Si NW branches, which is necessary to inhibit the effects of the large volume change (up to 370%) that can occur in Si during the charging/discharging process in a Li-ion battery, which leads to poor cycling performance due to the pulverisation of the active material and loss of electrical contact. The technique also does not allow any control of the density of the Si NW branches. Furthermore, this technique utilises gold as a catalyst. However gold is expensive, its contribution to the capacity of an electrode is negligible, and an excess of gold on the NWs has been shown to cause large irreversible capacity loss, poor performance and considerable capacity fade in a Li-ion electrode.
The paper entitled “Epitaxial core-shell and core-multishell nanowire heterostructures”, published in the Nature publication vol. 420, no. 6911, 7 Nov. 2002 discloses the synthesis of Si or Ge NWs using a gold catalyst deposited on the surface of an oxidised Si wafer. The NWs are grown using either silane or germane as Si or Ge precursors respectively. The Si shell is then deposited in a high pressure system using the same precursor. The heterostructure formed by this technique results in a NW having a long straight morphology, which results in little interconnection between neighbouring NWs and branches, which, as explained above, is desirable in order to provide a Li-ion battery of good performance. Furthermore, this synthesis technique is complex, and thus of high cost and low throughput. The use of high pressures and highly flammable precursors at high temperatures also makes the system described in this paper inherently very dangerous.
It is an object of the present invention to provide a material suitable for use as an electrode in a lithium ion battery which overcomes at least some of the above mentioned problems.
According to a first aspect of the present invention there is provided, as set out in the appended claims, a heterostructure for use as an electrode in a lithium ion battery, wherein the heterostructure comprises:
a nanowire stem of a first Li-alloying material and wherein a second Li-alloying material is distributed along the entire length of the surface of the nanowire stem.
The heterostructure may comprise a branched nanowire heterostructure.
The branched nanowire heterostructure may comprise the nanowire stem and a plurality of branches of the second Li-alloying material extending from the surface of the nanowire stem.
The heterostructure may comprise a core-shell heterostructure.
The core-shell heterostructure may comprise the nanowire stem and a shell of the second Li-alloying material provided on the surface of the nanowire stem.
The nanowire stem may be in direct contact with a current collector.
The current collector may comprise one of stainless steel, copper, nickel, aluminium or platinum.
The first Li-alloying material may comprise Silicon, Si, and the second Li-alloying material may comprise Germanium, Ge.
The first Li-alloying material may comprise Germanium, Ge, and the second Li-alloying material may comprise Silicon, Si.
The Ge to Si mass ratio may be adapted to provide an electrode of a particular capacity and rate capability.
The Ge to Si mass ratio may comprise a higher Si content than Ge content to provide an electrode of higher capacity.
The Ge to Si mass ratio may comprise a higher Ge content than Si content to provide an electrode of higher rate capability.
The first Li-alloying material and the second Li-alloying material may comprise Silicon, Si.
The first Li-alloying material and the second Li-alloying material may comprise Germanium, Ge.
The present invention also provides a method for synthesising a heterostructure for use as an electrode in a lithium ion battery, the method comprising the steps of:
The step of depositing a second Li-alloying material on the surface of the nanowire comprises step of:
The method may further comprise selecting the size of the plurality of nanoparticles in accordance with the desired size of the nanowire branches.
The method may further comprise selecting the concentration of the nanoparticle solution in accordance with the desired density of the plurality of the nanowire branches.
The nanoparticles may comprise one of: Sn, Au, Al, Ag, In, Bi, Pb, Cu, Fe or Ni.
The step of attaching a plurality of nanoparticles to the nanowire further comprises the steps of:
The step of synthesising a nanowire may further comprise the steps of:
The first Li-alloying material may comprise Silicon, Si, and the second Li-alloying material may comprise Germanium, Ge.
The first Li-alloying material may comprise Germanium, Ge, and the second Li-alloying material may comprise Silicon, Si.
The first Li-alloying material and the second Li-alloying material may comprise Si.
The first Li-alloying material and the second Li-alloying material may comprise Ge.
The Ge nanowire may be allowed to grow for at least one minute.
The method may further comprise preheating the substrate to approximately 430 degrees Celsius.
The current collector may comprise one of stainless steel, copper, nickel, aluminium or platinum, and wherein the method further comprises pretreating the current collector by evaporating a catalyst onto its surface.
The molecular linker may comprise ethanedithiol, and wherein the step of covalently attaching the tin nanoparticles to the Ge nanowire may further comprise:
The method may further comprise washing the Ge nanowire with attached tin nanoparticles with hexane and drying with a nitrogen line.
The step of growing silicon nanowire branches from the Ge nanowire by means of the tin nanoparticles acting as a seed material may further comprise the steps of:
The method may further comprise applying the vacuum to the chamber for one hour.
The first temperature may be approximately 125 degrees Celsius.
The high boiling point solvent may comprise 7 mls of squalene.
The reaction temperature may be approximately 460 degrees Celsius.
The Si precursor may comprise phenylsilane.
The longer the predetermined reaction time, the greater the content of Si may be in the heterostructure.
The predetermined reaction time may be determined based on the required Ge to Si mass ratio.
The method may further comprise setting the reaction time such that the Ge to Si mass ratio comprises a higher Si content than Ge content to provide an electrode of higher capacity.
The method may further comprise setting the reaction time such that the Ge to Si mass ratio comprises a higher Ge content than Si content to provide an electrode of higher rate capability.
The step of depositing Si on the surface of the Ge nanowire may comprise depositing Si using a plasma-enhanced chemical vapor deposition machine.
The longer the predetermined time period, the greater the content of Si may be in the heterostructure.
The predetermined time period may be determined based on the required Ge to Si mass ratio.
The method may further comprise setting the predetermined time period such that the Ge to Si mass ratio comprises a higher Si content than Ge content to provide an electrode of higher capacity than rate capability.
The method may further comprise setting the predetermined time period such that the Ge to Si mass ratio comprises a higher Ge content than Si content to provide an electrode of higher rate capability than capacity.
In yet another embodiment the present invention provides a Silicon-Germanium, Si—Ge, heterostructure for use as an electrode in a lithium ion battery.
The present invention also provides a method for synthesising a silicon-germanium, Si—Ge, heterostructure for use as an electrode in a lithium ion battery, the method comprising the steps of:
The present invention also provides a method for synthesising a silicon-germanium, Si—Ge, heterostructure for use as an electrode in a lithium ion battery, the method comprising the steps of:
The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which:
ai and 7aii shows EDX elemental maps of Si and Ge respectively;
bi and 7bii shows EDX elemental maps of Si and Ge respectively; and
The present invention discloses a heterostructure which is suitable for use as an electrode in a lithium ion battery. The heterostructure comprises a nanowire stem of a first Li-alloying material. A second Li-alloying material is distributed along the entire length of the surface of the nanowire stem.
Two embodiments for synthesising the heterostructure of the present invention are described in detail below, for the exemplary embodiment where the first Li-alloying material is Silicon and the second Li-alloying material is Germanium. However, it should be appreciated that this heterostructure could equally well be synthesised using many other techniques.
In accordance with the first embodiment for synthesising the heterostructure of the present invention, the heterostructure is synthesised as a branched NW heterostructure comprising a Ge stem contacted directly to the current collector, with Si branches growing from the surface of the Ge.
In step 100, a current collector is obtained by evaporating a catalyst onto a substrate, which in this embodiment is stainless steel. However, it should be appreciated that the current collector could equally well be other metals, such as for example copper, nickel, aluminium or platinum. In this embodiment, this involves evaporating copper 99.99% Cu in a glovebox based evaporation unit, with the Cu having a thickness of between 1 and 20 nm. However, many alternative catalysts could equally well be used, such as for example Sn, Au, In, Bi, and Ni. In step 105, the pretreated current collector is placed vertically onto a preheated substrate in an inert atmosphere. The substrate is preheated to approximately 430° C. A stainless steel confiner is then placed over the substrate, to prevent the escape of a Ge precursor. In step 110, a Ge precursor is delivered to the substrate and the reaction is allowed to proceed for at least 1 minute. In this embodiment, the Ge precursor comprises diphenylgermane, and the delivery is provided by means of an injection using a microliter syringe through a small hole in the top of the confiner. However, any other suitable Ge precursor could be used, such as for example triphenylgermane, triphenylgermane/phenyl silane mixture, phenylgermane, trichlorogermane, tetraethylgermane, tetramethylgermane or trigermane.
The reaction allows the Ge NWs to grow from the current collector via either a vapour-solid-solid mechanism or a vapour-liquid-solid mechanism, depending on the catalyst used. In step 115, the reaction is terminated by removing the confiner and allowing the Ge precursor to escape through evaporation.
In step 120, tin nanoparticles, (Sn NP) are synthesized through any known procedure. It should be noted that the plurality of Si branches which will be grown from the surface of the Ge NWs will be proportional to the size of the tin nanoparticles. In addition, the concentration of the Sn nanoparticle solution determines the density of the plurality of the Si branches which will be grown. Thus, the size and the concentration of the tin nanoparticles to be synthesized is selected in accordance with the desired size and density of the plurality of the Si branches to be grown from the surface of the Ge NWs.
Once the Sn NPs are synthesised, the NPs are washed and a ligand exchange performed (step 125). This is achieved in accordance with the present embodiment by adding 1 mL of hexane, 1 mL of Octadecene and 5 drops of Oleic acid to 1 mL of the Sn NP dark brown reaction mixture. The resulting mixture is then mixed thoroughly, such as for example through the use of a vortex mixer, and the solution allowed to settle into two separate layers (step 130). In step 135, the bottom colourless layer is discarded, while the brown top layer containing the Sn nanoparticles is recovered. This procedure is then repeated once more, and the brown solution containing the Sn NPs recovered and used without further purification.
In step 140, the Sn NPs are covalently attached to the Ge NWs. This may be achieved by immersing the Ge NW substrates in ethanedithiol (EDT) for approximately 1 hour. The substrates are then removed and rinsed, dried, and immersed in the Sn NPs solution. In accordance with this embodiment, the immersion in the Sn NP solution is for approximately 24 hours. The substrates are then washed thoroughly with hexane and dried with a nitrogen line.
In order to grow the Si branches from the surface of the GE NWs, the SN NPs are used as secondary seeds for Si NW growth. This involves a reaction being carried out between a Si precursor and the Sn NPs on Ge NWs. This reaction is carried out by placing the Ge NW with Sn NPs substrates vertically in a chamber. This chamber may be a round bottomed Pyrex flask which is attached to a schlenk line setup to provide a vacuum to the chamber via a water condenser. In step 145, a high boiling point solvent is added to the chamber and the temperature of the system is then increased to its reaction temperature. By holding the solvent under vacuum and increasing the temperature of the chamber, the solvent is de-gassed and any impurities are removed from it. In accordance with this embodiment, 7 mls of the solvent squalane is added and the temperature is increased to 125° C. through the use of a three zone furnace. A vacuum of at least 100 mTorr is then applied to remove moisture from the system (step 150). This vacuum is applied for approximately 1 hour. Following this, the system is purged with argon (Ar) (step 155). By purging the system with argon, an inert environment is provided in which the reaction can take place. In step 160, the temperature of the chamber is increased to the reaction temperature of approximately 460° C. under a constant flow of argon gas, to achieve a reflux (step 160). A water condenser is used to control the solvent reflux and ensure that the reaction is kept under control. In step 165, once the system has reached the correct reaction temperature, the Si precursor is injected into the chamber. In the described embodiment, the Si precursor comprises phenylsilane and it is injected through a septum cap into the chamber. The reaction is then allowed run for a predetermined time period, depending on the desired Ge to Si mass ratio (step 170). It will be appreciated in this regard that the longer the reaction time, the greater the concentration of Si in the resulting heterostructure. To terminate the reaction, the furnace is opened and the setup allowed to cool to room temperature before extracting the substrates.
The resulting heterostructure comprises a GE NW with Si branches distributed along the entire length of the surface of the Ge NW. As previously mentioned, the size of the Sn nanoparticles used for secondary seeding can be controlled in the process of the present invention, which in turn dictates the diameter of the Si branches. This is very desirable when the resulting heterostructure is being used in a Li-ion battery, in order to inhibit the deleterious effects of the volume change which occurs during the charging/discharging process in a Li-ion battery, which leads to poor cycling performance due to the pulverisation of the active material and loss of electrical contact. Thus, the present invention enables the NW diameter to be kept at a minimum, and thus reduces the negative effects of volume expansion.
In this embodiment of the invention where the single electrode is fabricated from a combination of Si and Ge materials, the electrode can benefit both from the excellent rate characteristics of Ge, as well as the high capacity and lower cost of Si. In addition, such a structure enables the cycling properties of the electrode to be tailored to the end application. Thus for example, if an electrode is being fabricated for an application requiring higher capacity than rate capability, it can be fabricated with higher Si content than Ge content. Conversely, if an electrode is being fabricated for an application requiring a higher rate capability than capacity, it can be fabricated with a higher Ge content than Si content.
High resolution TEM images of the highlighted areas of the Ge NW and the Si branch of
In accordance with the second embodiment for synthesising the heterostructure of the present invention, Ge NWs are grown directly from the current collector with a Si shell deposited on their surface.
Steps 400 to 415 of this embodiment involve the synthesis of Ge NWs, and are identical to steps 100 to 115 described in relation to the first embodiment. Once the Ge NWs are synthesised, Si is deposited on the surface of the Ge NWs such that it is distributed along the entire length of the surface of the Ge NW (step 420). In this embodiment, this is achieved through the use of a plasma-enhanced chemical vapor deposition machine. As was the case in respect of the first embodiment, the desired Ge to Si mass ratio is obtained by varying the deposition time, with a longer deposition time resulting in a higher Si content.
This process of the second embodiment of the invention results in the creation of a tortuous core-shell heterostructure. During use as an electrode, this tortuosity promotes the formation of a stable porous network of active material, and thus leads to a Li-ion battery having good performance characteristics.
While the described embodiments of the invention illustrate the process of the present invention for synthesising a Si—Ge NW heterostructure, it should be appreciated that the process of the present invention could equally well be applied in order to synthesise NW heterostructures comprising any other suitable Li-alloying materials. For example, the process of the present invention could be applied to synthesise a NW heterostructure comprising a Si stem having either Ge branches or a Ge shell, or a NW heterostructure comprising a Si stem having either Si branches or a Si shell, or indeed a NW heterostructure comprising a Ge stem having either Ge branches or a Ge shell.
The electrochemical properties of the branched NWs synthesised in accordance with the first embodiment of the invention may be illustrated by galvanostatic cycling in a two electrode Swagelok cell cycled in the voltage range of 0.01 to 1.0 V versus Li/Li+ in a 1 M LiP F6 in EC/DMC (1:1 v/v)+3 wt % VC electrolyte, and evaluating the electrochemical properties of three different Ge to Si mass ratios, such as 2:1, 3:1 and 4:1 Ge to Si, with each having maximum theoretical specific capacities of 2115, 1933 and 1823 mAh/g respectively. The electrochemical processes occurring during charging and discharging the material at a C/5 rate for 100 cycles can be shown by plotting the differential capacity of the 1st and 2nd cycles of each composition, which shows the dual cycling of both the Ge NW and the Si branch. The discharge capacities and Coulombic efficiencies (C.E) for each composition cycled at a C/5 rate are shown in
The rate capability of the branched NWs may be evaluated by charging and discharging the material for 5 cycles at rates of C/10, C/5, C/2, C, 2C, 5C, 10C and then back to C/10 (
It will be appreciated that the method of the first embodiment of the present invention ensures that branch growth occurs along the entire length of the NW stem. As a result, it provides a highly interconnected branched NW structure. During use as an electrode, the cycling of this highly interconnected branched NWs causes the material to restructure into a stable porous network of ligaments of active material due to a phenomenon known as lithium assisted electrochemically welding. Thus, the composition of the material is transient in nature, transforming from a heterostructure to a Si—Ge alloy as a consequence of repeated lithiation and delithiation.
There are numerous advantages to an electrode incorporating the heterostructure of the present invention. Through the use of the secondary seeding technique of the invention, the branch density may be controlled, by varying the concentration of the nanoparticle colloidal solution. In addition, for the embodiment where the heterostructure is a Si—Ge heterostructure, the Si to Ge mass ratio in the electrode is controllable. Through the increase of the Ge content, the rate capability of the electrode is improved, thereby adapting the electrode for applications that require quick bursts of energy, such as for example power tools. Conversely, through the increase of the Si content, the electrode can display a higher capacity but lower rate capability, and therefore be adapted for applications that do not require very high discharge rates, such as mobile phone.
Furthermore, when used as an anode in a lithium-ion battery the NWs overcome the problem of pulverisation encountered by bulk electrodes previously mentioned. This is due to the fact that they undergo a complete restructuring of their morphology to form a porous network of interconnected alloy ligaments. This network remains well contacted to the current collector despite the restructuring process, and can accommodate the mechanical stresses associated with the charge and discharge process without further deformation. As a result of this, the electrode exhibits a very stable capacity over its lifetime and is mechanically robust. High capacities of ˜2000 mAh/g are readily achievable, well above the maximum theoretical capacity of commercially available graphitic materials (372 mAh/g).
In addition, when Sn is used as the secondary seed, it contributes to the overall capacity of the electrode, due to the fact that Sn reversibly alloys with Li during the charge/discharge process.
The present invention also provides a simple and scalable low energy process for NW heterostructure growth.
The SEI layer formed on the Si—Ge alloy network is also much more stable than that formed on Si NWs alone when used in conjunction with the electrolyte additive, vinylene carbonate. This means that the cycle life of the heterostructure electrode is much improved over a Si NW electrode on its own.
It should also be appreciated that each NW is directly contacted to the current collector, due to the fact that a vapour-liquid-solid growth mechanism is used to synthesise the material. This eliminates the need for binders and conductive additives, which lower the overall specific capacity of the anode. The mixing, coating, pressing and drying steps involved in electrode preparation using traditional slurry coating methods are eliminated making the process more efficient. In addition, the method of the present invention is more environmentally friendly, as the use of harsh solvents which are toxic, expensive and generate flammable vapours during processing are not required.
The embodiments in the invention described with reference to the drawings comprise a computer apparatus and/or processes performed in a computer apparatus. However, the invention also extends to computer programs, particularly computer programs stored on or in a carrier adapted to bring the invention into practice. The program may be in the form of source code, object code, or a code intermediate source and object code, such as in partially compiled form or in any other form suitable for use in the implementation of the method according to the invention. The carrier may comprise a storage medium such as ROM, e.g. CD ROM, or magnetic recording medium, e.g. a floppy disk or hard disk. The carrier may be an electrical or optical signal which may be transmitted via an electrical or an optical cable or by radio or other means. In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa. The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.
| Number | Date | Country | Kind |
|---|---|---|---|
| 15170563.9 | Jun 2015 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/062415 | 6/1/2016 | WO | 00 |
