1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 Orthorhombic Nanostructured Li 2 MnSiO 4 /Al 2 O 3 Supercapattery Electrode with Efficient Lithium-Ion Migratory Pathway Miranda M. Ndipingwi, [a] Chinwe O. Ikpo, [a] Ntuthuko W. Hlongwa, [a] Zolani Myalo, [a] Natasha Ross, [a] Milua Masikini, [a] Suru V. John, [a] Priscilla G. Baker, [a] Wiets D. Roos, [b] and Emmanuel I. Iwuoha* [a] A Li 2 MnSiO 4 /Al 2 O 3 nanocomposite (LMSA) was prepared as positive electrode material for aqueous supercapatteries by hydrothermal synthesis of Li 2 MnSiO 4 nanoparticles (LMS) fol- lowed by wet chemical coating with Al 2 O 3 . Scanning electron microscopy (SEM) mapping of the spherical LMSA nanoparticles indicated a homogenous distribution of the constituent atoms. Small-angle X-ray scattering (SAXS) measurements revealed that a prominent population of the nanoparticles show a center-to- center spacing of 7 nm. This is resulting in a large surface area accessible for the migration of Li-ions and efficient charge storage, leading to improved electrochemical performance as a supercapattery electrode. X-ray diffraction (XRD) and solid-state nuclear magnetic resonance spectroscopy (SS NMR) studies portrayed the orthorhombic (Pmn2 1 ) crystalline phase of the LMSA and LMS materials which provides a good migratory pathway for the Li-ions. The nanocomposite showed a high rate performance as a positive electrode in an aqueous super- capattery assembled with activated carbon as the negative electrode. The hybrid cell delivered a maximum specific capacitance of 141.5 F g 1 and a maximum specific power of 4020.8 W kg 1 with good cyclic stability and capacitance retention of 93.6 % after 100 cycles. These results the promising potential of the Li 2 MnSiO 4 /Al 2 O 3 nanocomposite as candidate for advanced supercapatteries. 1. Introduction Climate change and the depletion of fossil fuel reserves with the associated emissions of greenhouse gases have placed a serious demand on the sustainable development of society and the environment. This requires an urgent transition towards a low-carbon energy future based on renewable energy. Optimi- zation of electrochemical energy storage devices such as supercapacitors and rechargeable batteries have been identi- fied as one of the key solutions to facilitate the storage, conversion, harvesting and widespread distribution of renew- able energy from the infinite but intermittent energy sources such as solar and wind energy. [1–3] Supercapatteries, which combines the high power density of supercapacitors and the high energy density of rechargeable batteries have emerged as advanced hybrid energy storage devices with superior power and energy output. This hybrid device whose behaviour is analogous to that of a supercapacitor with a high energy capability uses a capacitor-like and battery-like electrode and thus takes advantage of both capacitive and faradaic charge storage mechanisms at either the material or device level. [2,4–6] The capacitive behavior can either be like electric double layer capacitors (with non-faradaic charge storage) or pseudocapaci- tors (with faradaic charge storage). Various hybrid systems based on the supercapacitor-battery design (anode//cathode) have been developed. Such include Fe 3 O 4 -graphene//graphene- porous carbon, [7] Li 4 Ti 5 O 12 //LiFePO 4 -activated carbon, [8] activated carbon//LiMn 2 O 4. [9] Du et al. [1] reported a hybrid supercapacitor based on activated carbon//Li 2 Ni 2 (MoO 4 ) 3 in 2 M LiOH electro- lyte, which delivered a high energy density of 36.5 W h kg 1 and power density of 420 W kg 1 with capacitance retention of 68 % after 10000 cycles. Also, Shao’s group [5] fabricated a super- capattery cell based on activated carbon//binder-free Co 3 (PO 4 )·8H 2 O and it delivered good specific capacitance of 111.2 F g 1 , specific energy of 29.29 W h kg 1 and specific power of 468.75 W Kg 1 in 1 M NaOH aqueous electrolyte. Even though organic electrolytes provide a larger electrochemical window which impacts the energy capability of the hybrid systems, the safety concerns arising from their toxic and flammable nature coupled with the high cost is not satisfactory for high power applications. [10] Whereas aqueous electrolytes present several advantages such as high ionic conductivity, none or negligible volatility, greater safety and lower cost due to the simpler assembly process and the inexpensive and readily available water and water-soluble salts. Hence, with a potentially high performance cathode material, the aqueous electrolytes can be chosen as competitive candidates for supercapatteries. [10.11] [a] M. M. Ndipingwi, C. O. Ikpo, N. W. Hlongwa, Z. Myalo, Dr. N. Ross, Dr. M. Masikini, Dr. S. V. John, Prof. P. G. Baker, Prof. E. I. Iwuoha SensorLab Department of Chemistry University of the Western Cape Bellville, 7535, Cape Town, South Africa E-mail: eiwuoha@uwc.ac.za [b] Prof. W. D. Roos Department of Physics University of the Free State Bloemfontein, South Africa 223 Batteries & Supercaps 2018, 1, 223 – 235  2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Articles DOI: 10.1002/batt.201800045