...use a LC network to store small amount of electrical energy (500 mWHr or less)
You might think that 0.5 WHr is "small" but for an LC tank, it is HUGE.
So what is 0.5 WHr?
0.5 Watt = 0.5 Joule / second
One hour is 3600 seconds so that gives a total energy of 0.5 J/s * 3600 s = 1800 Joule
You want to use an LC resonator. In an LC resonator the energy resonates between a capacitor and an inductor.
Let's keep it simple and assume that at a certain time all energy is stored in the capacitor and the inductor is completely discharged.
What value does our capacitor need to be and to what voltage will it be charged to store 1800 J ?
I played around here and here are some solutions:
a 90 mF (milli Farad so 90000 uF) capacitor charged to 200 V
a 1 Farad capacitor charged to 60 V
a 400 Farad capacitor charged to 3 V
Of these the most practical solution is the last one, 100 F 3 V supercapacitors are available. However these capacitors only work with positive voltages. We could solve that by using placing capacitors in anti-series with protection diodes. Then we need 16 capacitors already !
Charging them will take time, you cannot "quickly" (few seconds) charge a super capacitor. Charging will not be much faster than charging a conventional battery. Source: Julian Ilett's Youtube channel, he sometimes messes around with super capacitors.
You could try to build a "better" capacitor and go for the 90 mF, 200 V option. I fear that the capacitor will need to be as large as a washing machine or a truck. Is that practical?
I'm not even going into the inductor as these are even less effective in storing energy in a given volume. You might need an inductor of the same size as a house.
I really think a battery is much more practical.
You could store less energy, less than the 0.5 WHr but then you get in the range where using a battery becomes even more efficient.