Nanoarchitectures for lithium-ion batteries

Efforts in lithium-ion batteries research have been to improve two distinct characteristics: capacity and rate. The capacity of the battery to store energy can be improved through the ability to insert/extract more lithium ions from the electrode. Electrodes capacities are compared through three different measures, based upon capacity per weight, gravimetric capacity, capacity per volume or volumetric capacity, and area normalized specific capacity, areal capacity. Also efforts have focused on improving the rate of discharge, which is based upon the mass and charge transport, electronic and ionic conductivity, and electron-transfer kinetic; easy transport through shorter distance and greater surface area improve the rate performance of the battery.^[1]

Anodes of carbon were chosen because of the ability of lithium to intercalate without excess volumetric expansion. High volumetric expansion causes degradation of the battery and a large amount of irreversibility rendering the battery useless for any application with a need for rechargeable energy storage. However the small amount of lithium-ion intercalation into carbonaceous electrodes limits the ultimate capacity of batteries, carbon based anodes have a gravimetric capacity of 372 mAh/g for LiC₆ ^[2]

Silicon has been considered as electrode materials because of its ability to have larger amounts of lithium-ion intercalation; the capacity of these batteries is approximately ten times greater than carbon. Considering that that atomic radius of Si is 1.46 angstroms and the atomic radius of Li is 2.05 angstroms, the formation of Li_3.75Si during intercalation of the lithium will cause a significant volumetric expansion of the host material ^[3]. One approach to solve this problem was to create composite anodes of silicon with materials that are less reactive with lithium to limit destruction of the electrode; however, the inclusion of other materials limits the performance of the material ^[1]. By reducing the size to the nanoscale numerous advantages are gained. At the nanoscale improved cycle life has been achieved, minimization of the size of silicon particles within a film of conductive binder to create films of active components that are below the critical flaw size minimizes crack propagation and failure ^[2][4]. A reduction in transport lengths also improves efficiency by reducing ohmic losses. The high surface area at the nanoscale improves the charge and discharge rate of the battery; this is due to both an increase in the electrochemically active area and a reduction in ionic and electronic transportation lengths. However, the increase in surface area to volume ratio at the nanoscale also leads to increased side reactions of the electrode with the electrolyte causing higher self-discharge, poor cycling lifetime, and lower calendar life. Recent work to improve the performance has been focused on determining materials that are electrochemically active within the range where electrolyte decomposition or side reaction of the electrolyte with the electrode do not occur ^[1].

Nanostructured architectures

For all the advancement of batteries within the past couple decades, a significant majority of battery designs are two –dimensional and rely on layer-by-layer construction ^[5]. Recent research has taken the nanodes into fully three dimensional structures. Through novel architectures the nanoscale benefits are maintained while the battery is scaled up. This allows for significant improvements in battery capacity; a significant increase in areal capacity occurs between a 2d thick film electrode and a 3d array electrode ^[6].

Three dimensional thin–films

Solid state batteries are the most similar geometry to traditional thin-film batteries; three dimensional thin-films use the third dimension to increase the electrochemically active area in the battery. Since thin film two dimensional batteries are restricted to between 2-5 micrometres the areal capacity of the device is significantly below that of three dimensional geometries.

One approach achieves this through a perforated substrate. The perforations in the sample were created through inductive coupled plasma etching on silicon^[7]. Another approached used highly anisotropic etching of a silicon substrate through electrochemical or reactive ion etching to create deep trenches. The requisite layers, an anode, separator, and cathode, for a battery were then added by low-pressure chemical vapor deposition. In this geometry the battery consists of a thin active silicon layer separated from a thin cathodic layer by a solid-state electrolyte. Through this approach the electrochemically active area of the battery is significantly increased, and thus thin-film of silicon nanoparticles (50 nm) that remains below the critical size for crack propagation can provide enhanced capacity and reversibility ^[8]

Interdigitated electrodes

Another three dimensional battery architecture is a periodic grouping of anodic and cathodic poles. For this design the power and energy density of the battery is maximized by minimizing the separation between the anodes and cathodes; an innate non-uniform current density occurs, however, and will cause lower cell efficiencies, poorer stability, and non-uniform heating within the cell. Relative to a two dimensional battery the length scale that over which transport must occur is decreased by 350% which not only improves kinetics but also reduces ohmic loses. Furthermore optimization of L can lead to a significant improvement in areal capacity; an L on the size scale of 500 micrometres results in a 350% increase in capacity over a comparable two dimensional battery. However, ohmic loses will increase as L increase, eventually offsetting the enhancement achieved through increasing L. For this geometry, four main designs were originally proposed: rows of anodes and cathodes, alternating anodes and cathodes, hexagonal packing of 1:2 anodes:cathodes, and alternating anodic and cathodic triangular poles where the nearest neighbors in the row are rotated 180 degrees. The design using rows has a large non-uniform current distribution. The alternating design exhibits better uniformity than the row design since there are a high number of electrodes of opposite polarity. For systems with an anode or cathode that is sensitive to non-uniform current density non-equal numbers of cathodes and anodes can be used; the 2:1 hexagonal design allows for a uniform current density at the anode but has a severely non-uniform current distribution at the cathode. The performance of the battery can be increased through changing the shape of the poles; the triangular design improves cell capacity and power by sacrificing current uniformity ^[9]. A similar system uses interdigitated plates instead of poles ^[5].

Concentric electrodes

The concentric cylinder design is a similar architecture to the interdigitated poles. Instead of discrete anodes and cathodes poles, the anodes or the cathode is kept as a pole which is coated by electrolyte. The other electrode serves as the continuous phase in which the anode/cathode resides. The main advantage of this system is that the amount of electrolyte is reduced, and thus a higher energy density than for the interdigitated system is achieved. This design maintains short transport distance like the interdigitated system and thus has a similar benefit to charge and mass transport while minimizing ohmic loses ^[5].

Inverse opal

A version of the concentric cylinder has been achieved through the packing of particles or close-packed polymer to create a three-dimensionally ordered macroporous (3DOM) carbon anode. In practice this system is fabricated by using colloidal crystal templating, electrochemical thin-film growth, and soft sol–gel chemistry. 3DOM materials have a unique structure of nanometer thick walls that surround interconnected and closed-packed sub-micrometer voids. When the 3DOM structure is coated with a thin polymer layer and then filled with second conducting phase, a lithium-ion battery can be formed. This method is advantageous because it leads to a lithium-ion battery with short transport lengths, high ionic conductivity, reasonable electrical conductivity, and removes the need for additives that do not contribute to the electrochemical performance of the battery. Performance of these devices can be improved through the coating of the sample with different materials. Coatings on the inverse opal structure can be used to improve the performance of the battery; the system has been coated with tin oxide nanoparticles to enhance the initial capacity ^[10]. Batteries created by this method rely on the infiltration of the network formed by the 3DOM structure to produce uniform coatings.

Nanowires and nanotubes

Nanowire and nanotubes have been integrated with numerous components of lithium-ion batteries. The reason for this interest in nanotubes and nanowires is because of the short transport lengths at the nanoscale, resistance to degradation, and ability to store lithium ions. For CNTs, lithium-ions can be stored on the exterior surface, in the interstitial sites between the nanotubes, and on the interior of the nanotube ^[11].

Numerous approaches have incorporated nanowires into lithium-ion batteries. The nanowires have been incorporated into the matrix with the anode and the cathode to create an anode / cathode with a built in conductive charge collector as well as enhancing capacity. The nanowires were incorporated into the anode / cathode through a solution based method that allows the active material to be printed on a substrate. The hope is that this architecture will allow for portable, lightweight, and disposable energy storage ^[12]. Another approach uses a CNT-cellulose composite to form a lithium-ion battery. CNTs were grown on a silicon substrate by thermal-chemical vapor deposition and then embedded in cellulose. Finally in order to create the battery a lithium electrode is added on top the cellulose across from the CNTs. The result is the creation of a flexible lithium-ion battery^[13]. Recently Si nanowires have been fabricated on a steel substrate by a vapor-liquid solid growth method. These nanowires exhibited close to the theoretical value for silicon and showed only minimal fading after a 20% drop after the first cycle. This performance is attributed to the facile strain relaxation that allows for accommodations of large strains while maintaining good contact with the current collector and efficient 1d electron transport along the nanowire ^[14].

Aperoidic electrodes

Every design so far has resulted in a periodic structure; however, as discussed earlier a periodic structure leads to non-uniform current densities which lower efficiency and decreases stability. The aperoidic structure is typically made of either aerogels or ambigels that form a porous aperiodic sponge. Aerogels and ambigels are formed from wet gels; aerogels are formed when wet gels are dried such that no capillary forces are established while ambigels are wet gels dried under conditions that minimize capillary forces ^[15]. Aerogels and amigels are unique in that 75-99% of the material is ‘open’ but interpenetrated by a solid which is on the order of 10 nm resulting in pores on the order of 10 to 100 nm. Furthermore the solid is covalently networked and therefore is resistant to agglomeration and sintering. Beyond aperiodicity, the reason these structures are used in lithium-ion batteries is because the porous structure allows for rapid diffusion of material throughout the material, and the porous structure provides a high surface area for reactions. The fabrication of these batteries are through a coating the ambigel with a polymer electrolyte and then filling of the void space with RuO₂ colloids that act as an anode.^[16]

Conformal coating of nanoscale electrodes

Nanoscale architectures for lithium-ion batteries are still mostly in the development phase. Very few of the batteries studied were more than half-cell experiments; usually the experiment only tests the anode or cathode. As geometries become more complex the need to develop non-line-of-sight methods to in-fill the complex geometries with materials that will act as electrolyte and oppositely charged electrodes becomes essential. These batteries can also be coated with various materials to improve their electrochemical performance and stability. However, chemical and physical heterogeneity in the samples cause molecular level control to still be a significant challenge, especially since the electrochemistry by which energy storage is not defect-tolerant ^[16].

Layer-by-layer (LbL)

LbL is used to conformally coat within the 3d nanoarchitecture; through electrostatically binding a charged polymer to an oppositely charged surface the surface is coated with polymer. Repeated steps of oppositely charged polymer build up a well-controlled thick layer of polymer on the surface. Polyelectrolyte films and ultrathin, less than 5 nm, of electroactive polymers have been deposited on the have been deposited on planar substrates using this method. However, problems exist with the deposition of polymers in structures of complex geometries, e.g. pores, on the size scale of 50-300 nm that can result in defective coatings. A solution to this problem is to use self limiting approaches ^[16].

Atomic layer deposition (ALD)

Another approach to coating is ALD which coats the substrate layer-by-layer with atomic precision. The precision is due to reactions being confined to the surface containing an active chemical moiety that reacts with precursor; this limits the growth to one monolayer. This self-limiting growth is essential for fully coating the electrode since the deposition of the polymer does not inhibit the mobility of other polymeric units to non-coated sites. Thicker samples can be produced by cycling gases in a similar manner to alternating with oppositely charged polymers in LbL. In practice ALD may require a few cycles in order to achieve the desired coverage and can result in varied morphologies such as islands, isolated crystallites, or nanoparticles . As described previously morphology can alter electrochemical behavior and therefore must be carefully controlled ^[16].

Electropolymerization

Electropolymerization offers another approach to conformally coating architectures with a thin film, 10 to 100 nm, of polymer. The electropolymerization of an insulating polymer results in a self-limiting deposition as the active moiety is protected; the deposition can also be self-limiting if the polymer can block the solubilized monomer and prohibit continued growth. Through the control of electrochemical variables, polyaniline and polythiophene can be deposited in a controlled manner. Styrene, methyl methacrylate, phenols, and other electrically insulating polymers have been deposited on the electrode geometries to act as a separator that allows ionic transport but inhibits electrical transport to prevent shorting of the battery. Mesoporous manganese dioxide ambigels have been protected by thin films, 7-9 nm, of polymer such that the dissolution of the manganese dioxide in aqueous acid was avoided. In order to achieve uniform coatings the architecture must be wetted by the solution containing the monomer; this can be achieved through a solution which has a low viscosity or a similar surface energy to that of the porous solid. Furthermore, as the scale continuous to decrease and transport through the solid becomes more difficult, pre-equilibration is needed to ensure a uniform coating ^[15].

References

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